Background: Adolescents are increasingly at risks of HIV infection through high-risk sexual behaviors. This paper aimed to assess the effectiveness of a HIV/AIDS educational intervention among female adolescents in Tehran. Methods: A randomized controlled trial was conducted among high school girls aged 12-16 years studying in Tehran in 2016-17. The sample was selected using multistage random cluster sampling. Four schools per district were randomly selected using probability-proportional to size. Adolescents were randomly assigned to the experimental (n=289) and the control (n=289) groups. The theory of planned behavior (TPB) was the basis of both the intervention and the evaluation phase. Both experimental and control groups completed a questionnaire designed for HIV related behaviors for female adolescents (HBQFA) at baseline and after six months follow up. A theory-based educational program was implemented for the experimental group. Results: The mean age of participants was 14.1 years (SD=0.96). In the experimental group, significant improvements were shown in adolescents’ HIV knowledge (31.9%, 95% CI: 28.8-35.0), attitudes towards HIV (16.6%, 95% CI=14.4-18.8), subjective norm (16.8%, 95% CI=12.9-20.6) and perceived behavioral control (19.1%, 95% CI=16.2-22.1), perceived parental support (17%, 95% CI=13.8-20.2), behavioral intention to prevent HIV (19%, 95% CI=16.3-21.6), and HIV preventive behavior (17.3%, 95% CI=13.9-20.6) (p<0.001). Conclusion: Theory-based educational intervention on HIV/AIDS prevention can significantly protect adolescents from misconceptions, wrong attitudes and risky behaviors and unsupportive social environment that expose them to greater HIV risk. Health policy -makers are advised to consider effective training programs related to HIV/AIDS prevention behaviors in the school system.