Purpose: This investigation examined a model to predict body mass index in order to prevent later development of obesity among adolescents. The hypothesized model was developed based on the principle of Social Cognitive Theory that social influences predict perceptions of cognition as well as behavior. Method: The 5,770 adolescents, aged 14-18 who participated in this study came from 11 separate school districts in Busan and Kyungnam province of South Korea. The adolescent surveys were administered in a group setting at the participating school. Of the 5770 surveys, adolescents with more than 10 percent missing data in the entire survey were excluded. The final sample contained 4,527 cases. The adolescents were included 2,250 (49.7%) males and 2,277 (50.3%) females. Data was analyzed Pearson correlation coefficients using SPSS 12 program and structural equation model (SEM) using Maximum Likelihood estimation was employed using Amos 5.0. Results: Social support directly affects cognitive determinants. Cognitive determinants directly affect behavior determinants which directly affect BMI. Cognitive and behavioral factors mediated the relationship between social support and BMI (GFI= 0.984, CFI= 0.974, RMSEA=0.031). Conclusion: Social levels should be supportive to help adolescents have healthier behaviors. This support can be accomplished by providing frequent support to establish positive cognitive factors as the foundation of the solution to prevent overweight and obesity.