Background: Friendship with the opposite sex is a need for adolescence; however, this kind of friendship is prohibited according to the Islamic laws in our country, therefore, parents, school authorities and disciplinary force may have intense reactions with their adolescence. These reactions may lead to secret relationship which it naturally associated with anxiety, depression and nervousness in adolescence which in turn may lead to mental disorders. Education could change attitude and increase teenagers' knowledge towards a suitable behavior in friendship with the opposite sex.Materials and methods: It was a quasi experimental study that was performed on 90 girl students of two high schools (case and control groups) in Tehran district 14.They were selected by cluster randomly sampling. Data were gathered through questionnaires. Having collected the initial data, six sessions were run for the case group (each 90 minutes), then, all the subjects were requested to recomplete the questionnaire.Results: Results have revealed that in the pretest analysis, most of the subjects had a moderate attitude and knowledge, however, following the education, most of the cases acquired suitable attitude and knowledge (p<0.001), while the differences among controls were not statistically significant.Conclusion: Results demonstrated the necessity of regular effective educative programs for high school students in order to promote their knowledge and attitude towards the friendship with the opposite sex.