Children and adolescent's health is a priority in all countries because the future situation of every country is based on it. It seems that a fifth (or probably a higher number) of children and adolescents suffer from dull to moderate mental disorders in the world. The aim of this study is to investigate the effect of education by group discussion about the mental health preventive behaviors in13-year-old girls in district six of Tehran. The study was a quasi-experimental study. The research populations were 53 girls of second grade of state secondary school in district 6 of Tehran. They were chosen by cluster sampling method. The case students were educated for 11 sessions for 1 hour and by group discussion and making posters in groups, then, the samples were tested by a valid and reliable checklist in 2 stages. Before intervention, the average of preventive behaviors was 44.49 (SD=7.946) and 52.68 (SD=8/781) after intervention. After the educational intervention, paired t-test showed a significant difference between the average of preventive behaviors (P<0.001). Finally, it appears that education by group discussion about mental health preventive behaviors is effective in adolescent girls.