This study is aimed to discuss the effects of exercise intervention on elementary school students' fitness. The participants of this study are schoolchildren, who are lower than 25% percentile rank in the norm, from 7 elementary schools in Kaohsiung City. Also, students and their parents all agreed upon it. There were total 166 students participated in this exercise intervention training, which lasted for ten weeks, twice a week, and 60 minutes each time. All the participants had to take the test of health-related physical fitness before and after training. The data of the test was analyzed through SPSS version 17.0 and examined through paired-samples T-test. We set the level of significance at .05 (α = .05). The following is the outcome of this study. After exercise intervention of ten weeks, schoolchildren's flexibility, fitness of muscle, explosive strength and cardio-pulmonary endurance all reach the level of significance both on pretest and post test (p<.05). The conclusion of this study is that exercise intervention improves elementary school students' flexibility, fitness of muscle, explosive strength and cardio-pulmonary endurance evidently. It indicates that exercise intervention promotes physical fitness.