Introduction: Overweight and obesity are risk factors for development of chronic diseases. Self-efficacy beliefs are direct predictors of a wide range of health behaviors. This study aimed to determine the effects of an educational intervention based on self-efficacy on the preventive behaviors of overweight and obesity among secondary-school female students in Mashhad, Iran. Materials and Methods: For this research, a quasi-experimental study, with a before and after design was used for which 70 secondary school female students were selected by the step sampling method and randomly assigned to two groups (35 cases and 35 controls).The data was collected using the weight efficacy lifestyle questionnaire (WEL). Educational programs were implemented for the experiment group and results were evaluated after the intervention by SPSS software. Results: Before the intervention, the two groups did not differ significantly in terms of average scores of self-efficacy structures for control of eating behaviors in situations, associated with positive or negative emotions, availability of food, social pressure and physical discomfort (P>0.05). After the intervention, however average scores in all situations were significantly increased in the experimental group (P<0.001). Average scores in all situations in the control group were not significant after the intervention (P>0.05). Conclusion: An educational intervention training based on lifestyle self-efficacy models could efficiently improve adolescents’ self-efficacy in situations related to positive or negative emotions, availability of food, social pressure and physical pain, emphasizing the benefitial effects of implementing a self-efficacy strategy in an educational intervention program to promote nutritional behavior in adolescents.