Background: Lack of knowledge about sexual matters among adolescents’ leads to pre- marital unprotected sexual indulgence, results in sexual transmitted infections, including HIV/AIDS, illegitimate teenage pregnancy, sexual abuse, violence and exploitation. Aim of study: Is to evaluate the effect of sex education program on knowledge of secondary school girls in El- Beheira Governorate. Hypothesis secondary school girls who attend sex education program exhibits better knowledge than those who don’t attend it. Material and Methods: Experimental study design was assumed. Settings: The study was executed at five secondary schools for girls in El- Beheira Governorate that were selected by multistage random sample technique. Subjects: 120 girl students were chosen randomly, these were randomized into 60 girl students for study groups and 60 girl students for control groups. Tools: two tools were developed by the researchers for data collection. The first tool was concerned with Socio-demographic characteristics, and the second tool was to assess the knowledge of girl students before and after the sex education program. Results: Among study groups, the program created a positive effect on general information about sexuality (p= 0.001), anatomy and physiology of male reproductive system (p= 0.001), anatomy and physiology of female reproductive system (p= 0.214), fertilization/ conception (p= 0.042), and contraception (p= 0.417). Conclusion and Recommendations: After implementation of the sex education program, a positive improvement on knowledge of study group was apparent: So, this study recommended Sex education topics should be implemented in the curriculum of school and college, and teachers should actively participate in sex education program. Keywords: sex education, reproductive health, and sexuality