Prepared for his kickboxing career by Korean K-1 fighter Choi Hong-man, he debuted in K-1 fighting at the World GP 2005 event in Seoul in March, 2005, which he won in the final vs. Kaoklai Kaennorsing. In the Final Elimination he defeated Bob Sapp with a 2–0 decision, eventually losing to Remy Bonjasky in the World Grand Prix Final.
He got his nickname "Techno Goliath" (테크노 골리앗) when he was a ssireum wrestler in South Korea, as after a victory he would dance to techno music. He is referred to on Japanese television and news articles as the 'Korean Monster' (コリアン・モンスター).
He started the season 2006 at the K-1 World GP in Las Vegas with a unanimous decision win over Sylvester Terkay aka "The Predator". His new coach was the former Japanese Korean K-1 fighter Kin Taiei (anglicised to Taiei Kin).
One of Choi's most notable fights was against current K-1 World Champion Semmy Schilt at the World GP 2006 event in Seoul. The fight was unique due to the enormous size and weight of both fighters (Schilt 2.12 m (6 ft 11 in) 128 kg (282 lb), Choi 2.16 m (7 ft 1 in) 161 kg (355 lb)). Choi won the fight by 2-0 decision, but it was a disputed win.
On September 30, 2006, Choi fought Jérôme Le Banner in the final eliminations, and lost after 3 Rounds and 1 extra Round on points. After the fight, Le Banner said: "He is dangerous, his knees are already almost at the level of my head, he's not human! But he's a good guy and I like him, he's very tough, maybe the strongest guy in K-1, and he has hard bones -- when I kicked him, it hurt my leg! I'm sure with more experience, in two years no one will be able to knock him out!"
On March 4, 2007, he was knocked out for the first time in his career. Mighty Mo landed his trademark overhand right to the chin and Choi was not able to stand up again. In this fight he weighed 165.4 kg (365 lb; 26 st 1 lb), was very slow and many say it was his worst performance.
On August 5. at the Asia World Grand Prix in Hong Kong, he beat Gary Goodridge by KO at 1:34 in the first round. He weighed 163.5 kg (360 lb; 25 st 10 lb), the same when he fought Schilt in June 2006. Notable on this fight was his stance. Since his K-1 debut in 2005, he has fought using an orthodox stance. However in this fight, he adopted for the first time a Southpaw stance. This was a strategy by his coach Kin Tailei for an eventual re-match with Mighty Mo (to block his devastating overhand right better as in orthodox stance).
On September 29 at the K-1 Final Eliminations in Seoul, fighting southpaw stance again, he redeemed his previous loss and defeated Mighty Mo by decision. The win was disputed because Mo was able to land many hard hands on Choi´s head, while the Korean missed most of his shots. However, due to a kick Choi delivered to Mighty Mo's groin in the second round that was inexplicably ruled a knockdown. Mighty Mo was quoted in the post-fight interview: "He (Choi) seemed a little bit stronger because I hit him with some good shots and he didn't fall down so I was surprised, actually harder than the ones I hit him with before. He must have been practicing taking punches."
In the World Grand Prix Final 2007 tournament on December 8, he again lost by decision to Jérôme Le Banner. Choi was announced by Jimmy Lennon Jr. as weighing in at 166.6 kg (367 lb; 26 st 3 lb), to date his heaviest pre-fight weigh-in. In the post-fight Choi said: "He (Jérôme) has been my toughest opponent, who was able to resist my powerful punches and counter with speed, I admire his physical strength and I'm sure with more experience will be able to defeat him."
On September 27 at the K-1 final Elimination in Seoul, he came back to K-1 and fought against the current Heavyweight champion Badr Hari. After 3 rounds of fighting, Choi's corner threw in the towel due to a rib injury. Although Hari was unable to faze Choi with two heavy punches thrown in the first and second rounds, in the third round he began targeting Choi's ribcage with punches and kicks.
Since his last fight, Choi has lost much weight. He was announced with a weight of 149 kg (328 lb; 23 st 6 lb).
Choi was picked as a first reserve fighter against Ray Sefo at the 2008 K-1 Championship Finals which he lost by Unanimous decision.
On December 31, Choi easily won his first mixed martial arts fight on K-1 Dynamite in Osaka. He decisively beat former TV star Bobby Ologun in just 16 seconds in what was considered a huge mismatch.
Choi was scheduled to face former WWE, IWGP, and NCAA national champion amateur wrestler Brock Lesnar at the combined promotion event K-1 Dynamite!! USA, a pay-per-view event at the Los Angeles Memorial Coliseum, on June 2, 2007. However, reportedly due to a benign tumor on his pituitary gland, Choi was denied his California fighter's license on May 23, 2007, putting Dynamite!! USA's main event in jeopardy only 10 days before its scheduled occurrence. Choi was replaced in the fight by fellow Korean fighter Kim Min-soo.
He was confirmed to fight MMA heavyweight and former Pride heavyweight champion Fedor Emelianenko at Yarennoka on New Year's Eve. Choi landed twice on top of the two takedown attempts of Fedor. He escaped an armbar attempt on the first takedown, but after the second one was forced to tap to another armbar applied by Fedor at 1:54 of the first round.
Choi fought again in MMA against Mirko Cro Cop on December 31, 2008 in Sataima, Japan. Cro Cop defeated Choi with multiple kicks to the legs.
He fought retired baseball player Jose Canseco in an MMA match on May 26, 2009 for the promotion Dream as part of its Super Hulk Tournament. He beat Canseco in 1 minute 17 seconds. However, Choi would lose in the semi-finals to Ikuhisa Minowa via heel hook at Dream 11.
Choi was expected to participate in Dream 14, which was supposed to have taken place in Seoul, South Korea. But many factors, including the issue that many top Korean stars such as Denis Kang and him (Choi) were not available in April, contributed to the Korea event being canceled.
In July 2015, after 5 years and 9 months of inactivity in combat sport, he gave his comeback against Brazilian fighter Carlos Toyota, weighing 140 kg (310 lb), the lowest to this day. He lost via knockout in the first round (multiple punches to face and jaw). Toyota was the second fighter besides Mighty Mo who knocked out Choi with head punches.
In December 2015 he fought the Chinese Luo Quanchao and won by cut stoppage (TKO).
In April 2016, he fought the "Chinese" Aorigele, whom he defeated by knockout in round one. He was weighing 157 kg (346 lb)
In April 2008, Choi joined the Korean army for his 21 months military service. After failing two medical tests, he was relieved from his military duty. The main reason was a problem with the sight in his left eye, which was the result of a brain tumor. On June 9, the tumor was finally removed through surgery.
Choi recently began a singing career with supermodel Kang Soo Hee in Korea under the name of Beauty & The Beast. He also made a rap single featuring vocals by the model Kang Soo Hee. In 2009, he made his movie debut in the Japanese film Goemon.
He has some Tattoos. His surname "Choi" on his left shoulder., on his left forearm the phrase „Secret of success is constancy to purpose“., on his chest "No Pain No Gain“.
In April 2010, Choi appeared as part of the main cast in Japanese TV drama series Kaibutsu-kun, adapted from Fujiko Fujio's Kaibutsu-kun of the same name. Choi played the character Franken, which was one of the three understudies of prince Kaibutsu-kun.
In April 2012 he appeared as a guest in the 19th episode of the Korean variety show Invincible Youth 2.
He has also done several endorsement works with actress / former model Karina. He has been a face for Lotte's ice cream product Mona Oh ('Monaka-King') alongside Karina. He threw out the ceremonial first pitch along with Karina at the Chiba Lotte Marines-Yomiuri Giants game in Chiba Marine Stadium on June 11, 2006. He has also done endorsement works for videogame LittleBigPlanet with Mari Yaguchi, a former member of the music group Morning Musume.
In March 2012, he made a special guest appearance in episode 2 and another on July 2012 in episode 17 of JTBC variety programme Shinhwa Broadcast hosted by boyband Shinhwa.
On September 08th and 15, 2013, he appeared in episodes 310 and 311 of the variety show 2 Days & 1 Night (KBS2) as a guest of regular cast-member Kim Jong-min.
On November 16, 2014, he appeared in episode 28 of reality show Roommate (SBS) as a guest along with female singer Hong Jin-young.
ON June 23, 2015, he appeared in episode 254 of variety show Running Man (SBS) as a guest and as well as a hunter for Running Man nametags.K-1
2005 K-1 World Grand Prix in Seoul Champion
2009 DREAM Super Hulk Tournament Semifinalist
Road Fighting Championship
2016 Road FC Openweight Tournament Runner Up
41st Cheonhajangsa Ssireum Championship in 2003