The Tour de France started with 15 teams, of 10 cyclists each, from five different countries. A few days before the Tour started, it became known that Paul Gutty had failed a doping test when he won the French national road championship. Gutty was removed from his Frimatic team, and replaced by Rene Grelin.
The teams entering the race were:
After his dominating victory in the previous year, Merckx was the major favourite. The main competition was expected from Luis Ocaña and Bernard Thévenet. Early in the race, 86 journalists predicted who would be in the top five of the Tour. 85 of them expected Merckx to be in the top five; Ocana was named by 78, Poulidor by 73. Merckx had already won important races in 1970, including Paris–Roubaix, Paris–Nice, the Giro d'Italia and the Belgian national road championship. Luis Ocaña, who had won the Critérium du Dauphiné Libéré and the Vuelta a España, suffered from bronchitis, but still started the Tour, unable to seriously challenge Merckx.
The 1970 Tour de France started on 27 June, and had no rest days. After the financial success of the split stages in the 1969 Tour de France, even more split stages were used in the 1970 Tour.
The big favourite Merckx won the opening prologue, but he decided not to try to keep this leading position during the entire race. In the next stage, Merckx' team chased back all the escapes, so the stage ended in a bunch sprint, and Merckx kept the lead. In the second stage, a few cyclists escaped, and two of Merckx' team mates, Italo Zilioli and Georges Vandenberghe, joined the escape. Merckx' team mate Zilioli was ranked highest amongst the escaped cyclists, and none of them were considered competitors for the general classification, so Guillaume Driessens, Merckx's team leader, allowed the escape to work, and told Zilioli and Vandenberghe to give their best. Merckx however chased his own team mates. The group stayed away, Zilioli won the sprint and became the new leader, 4 seconds ahead of Merckx. After the stage, Merckx was angry at his team leader, because he had allowed Zilioli to "steal" Merckx' yellow jersey, but Driessens explained him that the other teams had spent energy to chase Zilioli, and the argument was over. Merckx team won the team time trial, and controlled the next stages, keeping Zilioli the leader with Merckx in second place.
In the sixth stage, Zilioli had a flat tire. Normally, if the leader in the Tour de France suffers a flat tire, a team mate would offer his wheel, and some team mates would stay with him to help him get back into the peloton. However, this time Merckx was considered more important, and Zilioli was given no help. Zilioli finished the stage one minute behind, and Merckx was the new leader.
The seventh stage was split in two. Merckx won the first stage with a solo break, and finished second in the second part, a time trial. In that time trial, run during the rain, Roger de Vlaeminck, third in the general classification, took too much risk, fell down and left the race in an ambulance. Merckx saw De Vlaeminck lying on the street during his race, and decided to take less risks, allowing José Antonio González Linares to win the stage by three seconds. Because Roger de Vlaeminck had left the race, his team Mars needed a new captain. Debutant Joop Zoetemelk was the highest ranked cyclist, and became the new captain.
In the ninth stage, Mogens Frey and Joaquim Agostinho, team mates, broke away together. They worked together to stay away, but near the end of the stage Frey stopped working and had Agostinho do all the work, even after his team manager told him to help. In the sprint, Agostinho expected his team mate to give him the victory because he had done all the work, but to his surprise Frey started to come around him. Agostinho then grabbed Frey's handlebars, and crossed the finish line first. The race jury did not allow this, and gave the victory to Frey, putting Agostinho in second place.
In the tenth stage, when the first medium mountains showed up, Merckx won the stage, and only three cyclists were able to stay with him, including Zoetemelk. Zoetemelk then rose to the second place, and he became the most important rival for Merckx.
Zoetemelk, along with Luis Ocana would be the two most important rivals for the remainder of Merckx's career.
Zoetemelk said that he would focus on defending his second place, because he thought Merckx was better than the rest of the world.
After the thirteenth stage, Merckx heard that Vicenze Giacotto, who started the Faema team around Merckx, had died of a heart attack.
Merkcx increased his lead steadily in the mountain stages in the Alps. After he won the stage to the Mont Ventoux, Merckx briefly lost consciousness.
In the two Pyrenéan stages, Merckx did not win. He was suffering from stomach problems, and changed bicycles several times. The young Bernard Thévenet won the first, showing his potential as a future Tour winner.
Merckx was the third cyclist to win the Tour-Giro double in one year; Fausto Coppi and Jacques Anquetil had done it before. Coppi and Anquetil were over thirty years old at their doubles, Merckx was only 25. The margin with the second placed cyclist was less than the year before; according to J.B. Wadley, the difference was that Merckx stopped attacking in 1970 after the Mont Ventoux; had he been inclined to win more time, he probably would have been able to.
There were several classifications in the 1970 Tour de France, three of them awarding jerseys to their leaders. The most important was the general classification, calculated by adding each cyclist's finishing times on each stage. The cyclist with the least accumulated time was the race leader, identified by the yellow jersey; the winner of this classification is considered the winner of the Tour.
Additionally, there was a points classification, where cyclists got points for finishing among the best in a stage finish, or in intermediate sprints. The cyclist with the most points lead the classification, and was identified with a green jersey.
There was also a mountains classification. The organisation had categorized some climbs as either first, second, third, or fourth-category; points for this classification were won by the first cyclists that reached the top of these climbs first, with more points available for the higher-categorized climbs. The cyclist with the most points lead the classification, but was not identified with a jersey in 1970.
Another classification was the combination classification. This classification was calculated as a combination of the other classifications, its leader wore the white jersey.
The fifth individual classification was the intermediate sprints classification. This classification had similar rules as the points classification, but only points were awarded on intermediate sprints. In 1970, this classification had no associated jersey.
For the team classification, the times of the best three cyclists per team on each stage were added; the leading team was the team with the lowest total time. The riders in the team that lead this classification wore yellow caps.
The intermediate sprints classification, sponsored by Miko, was also named "hot spot". The combativity award was given to Roger Pingeon. The new rider classification was first calculated in 1970. It is not the same as the young rider classification, introduced in 1975.
Merckx had been so dominant during the entire Tour, that the organisation was afraid the race would become dull. The director Félix Lévitan announced that rule changes were considered to break the power of Merckx's team, that he was considering to return to national teams, and to reduce the number of time trials in the Tour. The 1971 Tour did not see major changes in rules, but the number of individual time trials decreased from five to two.