This study was to investigate characteristics of power output during repeated sprint running efforts in high school boys who were participating in soccer as an extra curricular activity. Subjects were 12 high school boys (age=17.±0.2 yr, height=170.6±1.3 cm,body mass=59.7±1.1 kg,mean±SE) who were the members of a soccer club and 8 active young adult men (27.4±1.0 yr,height= 171.2 ±1.8 cm, body mass= 76.8±3.8 kg) who have been athletes in the past. The subjects repeated 10 maximal sprint running efforts for 5 seconds with an interval of 10 seconds between trials on a treadmill with a torque motor to neutralize the frictional resistance of treadmill belt.Mean force, velocity and power values during each sprint effort were determined. The mean force and power values in a trial span from the first to fifth effort were significantly higher in adult subjects than high school boys even when calculated in terms of per unit of body mass. Moreover, the adult subjects showed significantly higher mean velocity values during the trial span between the first and third efforts. However, there were no significant differences between the two groups in mean velocity values between the fourth and tenth trials and mean force and power, expressed as relative values to body mass, between the sixth and tenth trials. The high school boys showed significantly lower values than adults subjects in the percentage of decline in all variables with increasing trials. Thus, the present results indicated that the high school boys were inferior to adult subjects in power generation capability during the initial stage of the repeated maximal sprint running task. This might be due to the differences between high school boys and adult subjects in either anaerobic working capacity or movement pattern during sprint running efforts.