Purpose : This study was designed to apply a "stage of change" exercise intervention program to obese children in elementary school and to examine its effects on their BMI. stage of change. process . and self-efficacy in exercise. Methods : The experimental group included 64 children in the fourth . fifth and sixth grades. from one elementary school. whose BMI was in the H5th percentile or more for their age groups. The control group had 6 1 students from a different one elementary school whose conditions were the same as those of the experimental group. Results: After the stage of change exercise intervention was applied, the BMI of the experimental group decreased by a significant enough degree to conclude that the program was effective in decreasing BMI. After the application of the program. the experimental group experienced a significant increase in their helping relationships. counter-conditioning . reinforcement management and stimulus control compared 10 the control group. After the program. self-efficacy in terms of exercise increased significantly in the experimental group. Conclusion: A stages of change exercise intervention program has been shown to engage individual students and encourage them 10 maintain a higher fitness level that promote s self-efficacy in exercise and therefore moves them towards healthier lifestyles. Moreover. the study proved that a significant reduction of BMI was achieved by the stages of change exercise intervention program.