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BACKGROUND AND OBJECTIVE: QTc fluctuations, in rest and exercise lead to arrhythmias and sudden cardiac death. Caffeine is an energy source and it may moderate this bad fluctuation. The purpose of this study was to compare the effect of caffeine on QTc and blood pressure during isometric and isotonic contractions in athletic and non-athletic men. METHODS: in this semi-experimental study, 20 healthy men were randomly selected and placed in two groups of athletes and non-athletes. Pre-test: at first, each subject pressed the dynamometer handle at maximum power for 30s (isometric). After 25 min rest, the isotonic activity was performed in 15 repetitions of 5 seconds. At the end, each subject takes 400 mg of caffeine, and after 60min, the post-test similarly, was repeated as a pre-test. QTc changes and blood pressure were checked before and after the test. FINDINGS: The average weight of individuals was 76±13.93 kg. Caffeine in the athlete group resulted in a decrease in QTc during isometric and isotonic contractions of 11.5 ms and 15ms respectively. But in nonathlete groups, QTc during isometric and isotonic contractions were respectively 0.7ms increase and 4.1 ms decrease. There was no significant correlation between QTc in both groups, but diastolic blood pressure increased significantly (p=0.02). CONCLUSION: The results of the study showed that caffeine consumption had no significant effect on QTc interval but it affected diastolic blood pressure