Abstract: Objective To examine the effects of exercise duration and intensity on body composition among junior high school students,and to provide scientific basis for arranging the effective amount of exercise for junior high school students.Methois A total of 226 students from 6 classes participated this experimental trial.Different duration×intensity exercise elements were introduced into physical education class (40 min 3 times a week for 8 weeks),including 15,10 or 5 minutes vigorous and moderate exercise.Results The results showed no significant differences in the body mass index and waist-hip ratio pre-and post-experimental trial.Significant difference in somatic fat volume was only observed in group 6;for the fat free mass index,significant differences were found in group 2,group 3,group 4,group 5,group 6.The difference of PBF and PFFM was not statistically significant in each class students.For the lean muscle mass index,significant changes were observed in the experimental group 2,group 4,group 5 and group 6(P<0.05).There was significant differences in percentage of lean muscle mass index for experiment group 1,group 4,group 6(P<0.05).Conclusion Vigorous exercise for 10 min and moderate exercise for 15 min can increase percentage of fat free mass and decrease percentage of body fat.Vigorous exercise for 10 to 15 min and moderate exercise for 15 min during physical education class can help increase percentage of lean muscle mass.