While attending Yonsei University, Na was considered the best hitting pitcher in the South Korean collegiate baseball league. As a freshman in 2008 Na batted a team-leading .329 with 11 RBI and posted a 3–1 record and a 1.70 ERA with 83 strikeouts (runner-up in the collegiate league) in 74.0 innings pitched.
As a sophomore in 2009 Na led the collegiate league in home runs (4) as a cleanup hitter and amassed 7 wins (2 complete games – 1 shutout) with a 2.93 ERA as a starting pitcher. Also, he was runner-up in strikeouts (76) and third in RBI (23). After the season, he was selected for the South Korea national baseball team and participated in the 2009 Asian Baseball Championship as a designated hitter and relief pitcher.
As a senior in 2011, Na struggled a bit posting a career-worst 3.76 ERA, but fanned 68 batters. In September 2011, Na was called up to the South Korean national team again for the 2011 Baseball World Cup held in Panama.
Na Sung-bum originally was drafted by the NC Dinos as a pitcher. But he changed his position to outfielder because Kim Kyung-moon suggested that. In 2012, he played 94 games in the farm league, finishing with a .303 batting average, 16 home runs, and 67 RBI. He won RBI leaders and Home Run Title.
In his first full season the next year, when he hit 14 home runs with a .243 batting average and 64 RBI, Na was runner-up in the KBO Rookie of the Year voting, behind team fellow Lee Jae-hak.
Na broke out in 2014 when he hit 30 home runs, batted .329 and drove in 101 runs, and quickly became a favorite of the Dinos' organization and a household name among the team's fans, voted the top competition at the KBO All-Star voting. He garnered 968,013 votes from fans and another 193 votes from KBO coaches and fellow players to lead the 22 All-Star starters.
Na maintained his status as one of the league's elite power-hitting outfielders in 2015, when finished ninth in the league with a .326 batting average, drove in 135 runs (fourth in the league) and slugged 28 home runs (seventh in the league).
Despite getting off to a hot start on the 2016 season, cold streaks in the second half of the year ruined chances of Na putting together a great year. However, he finished the year with respectable offensive numbers, finishing the year with 22 home runs, 113 RBI and a .309 batting average.