PURPOSE: The purpose of this study was to identify the effects of a school-based obesity control program based on behavior modification and self-efficacy for obese elementary school children. The program was composed of strategies to modify diet and exercise habits and to increase self-efficacy. METHOD: The subjects were 57 obese children (experimental group = 28, control group = 29) whose Rohler index was 150 and over. The program was implemented once a week for 12 weeks from September 16 to December 12, 2003. The data was analyzed by Fisher's exact probability, chi2-test, t-test, and Wilcoxon Rank Sum test. RESULT: The Rohler index, fat mass and lean body mass of the experimental group positively changed after the intervention more than those of the control group, but there was a significant difference in the Rohler index only (t=2.06, p=.045). In addition, obesity stress significantly decreased (z=-2.86, p=.047) and dietary self-efficacy significantly increased (t=2.35, p=.023) in the experimental group than those of the control group. CONCLUSION: This study supports that a school-based obesity control program based on behavior modification and self-efficacy can be effective in decreasing obesity stress and increasing dietary self-efficacy. Parents, school nurses and the other support groups should be encouraged to participate from the planning stage of the program to be effective in weight control of obese elementary school children. Also school-based program should be implemented as an essential course in the curriculum, not as an elective.