Objective: To assess the effectiveness of sexual education intervention among seventh grade students, and to provide support tools for sex education for middle school students. Methods: Seventh grade students from six schools in Longnan, Gansu Province were randomly assigned to an intervention group (251 students) and a control group (222 students). After 8 weeks of school based sex education in the intervention group, the results were compared before and after the intervention using self assessment knowledge and attitude scales. Results: The knowledge score in the control group decreased by (2.46±1.21) in the follow up survey compared to the baseline survey. The intervention group scored (30.54±1.34) significantly higher than baseline ( t =22.76, P <0.01). After adjusting the sex ratio to 1∶1, the mean difference between the two groups after the intervention was (27.86±1.87) ( t =14.90, P <0.01). The interaction dit between time and intervention was (33.01±2.50) ( t =13.19, P <0.01) in difference analysis. The intervention effect size Hedge s g on knowledge in the intervention group was 1.27. The proportion of positive attitudes towards sex increased in the intervention group on 12 out of 14 questions, with percentage ranging from 7.5% to 25.9%. At the follow up, the improvement in attitudes towards 12 questions was substantial for girls and 8 questions for boys in the intervention group. The attitude effect size Hedge s g was 0.99 in the intervention group. Conclusion The implementation of school based sex education is capable of significant improving students sexual knowledge and attitude in the short term.