Background: Weight loss and decreasing the Body fat percentage (BF%) is motivated to optimize performance. In order to achieve these, many supplements are used by athletes, however the possible negative or synergic effects have not been fully described in the literature, specifically in humans. The present study was conducted to investigate the co-administration effects of two common used supplements in body weight and BF% management to recommend athletes for safe weight and BF% reduction. Materials and Methods: In the present double-blind, randomized, parallel, placebo-controlled study, the effect of six-week co-administration of caffeine and carnitine was determined on changes in body weight (BW), BF%, serum leptin concentration and lipid profile (triglyceride, HDL Cholesterol, LDL Cholesterol and Total Cholesterol), fasting blood glucose (FBG), and free fatty acid (FFA) changes. Twenty eight male teen soccer players from Ahvaz-Iran, were divided in three groups (group CafPlc, caffeine (6 mg/kg/day) + dextrose; group CafCar, caffeine (6 mg/kg/day) + carnitine (2g); and group Plc, dextrose). Results: Caffeine-carnitine had a lowering effect on BW (P=0. 02) and BF% (P=0. 03), compared to caffeine alone and placebo in male teen soccer players (mean age of 16. 92 ± 0. 76 years). TG was significantly decreased in CafCar (P=0. 04). FFA levels were increased in CafCar (P=0. 04) and there was significant differences between CafCar and Plc groups (P=0. 01). FBG was increased in both CafPlc and CafCar (P=0. 01 and P=0. 02, respectively), with no significant differences between groups. Conclusion: The synergistic effect of caffeine-carnitine might be suggested to decrease the BF% and BW, besides it may prevent the increment of FFA levels; however it should be prescribed cautiously since it increased FBG levels.