Background: Overweight and obesity are the most serious public health challenges of developed and developing countries. The prevalence of childhood obesity has increased worldwide. This study aims to identify the behavioral determinants of overweight and obesity among adolescents. Methods: This is a qualitative study with directed approach to content analysis of in-depth semi-structured interviews. A total of 27 obese and overweight students (11girls, 16 boys) were recruited in this study based on the PRECEDE model (predisposing factor). The participants were recruited using a purposive sampling method to reach the maximum variation sampling. This sampling took over 6 months in 2018 at the Ghaemshahr middle school. Content analysis approach (directed) was used to analyze the interview. Results: Five main themes and 20 sub-themes of behavioral determinant were emerged from the interview. The main themes included perceived benefit, perceived threaten, self-efficacy, cues to action, and locus of control. The subthemes included self-empowerment, motivation and diligence role, enough information and parent pressure, long term benefits, physical health, doing favorite sports, perceived susceptibility and perceived severity, and internal and external locus of control. Conclusion: Self-efficacy and perceived benefit are the most significant determinants of preventive behavior in obese and overweight students; therefore, considering these variables in students can be applied in designing an interventional program that affect obesity preventive behaviors, increase parents’ awareness and school authorities.