Applying the appropriate assertiveness training program is necessary to help students to know the fine line between assertion and aggression, change their negative attitudes, express their positive and negative emotions without violating the rights of others Aim: To evaluate the effect of an assertiveness training program on aggressive behavior among preparatory school students Design: A quasi-experimental research design was used. Setting: The study was conducted at Huda Sharawy preparatory school, El-Shaheed Ahmed Hamdy preparatory school for Girls, 25 January preparatory school and Mahatet El- kahraba preparatory school, Helwan, Cairo. Sample: A simple random sample of 176 students was selected from 1760 preparatory school students at 1st grade in El-Masara district during the academic year 2021-2022. Tools: Data were collected by using the following tools: Socio-demographic characteristics questionnaire, Aggression questionnaire and Assertiveness formative questionnaire. Results: there were statistically significant differences between pre and post training program regarding total aggression and assertiveness among the studied students. 62.5% of the studied students had mild level of total aggression pre assertiveness training program while improved to 82.4% of them post training program. While, 9.6% of them have high assertiveness pre training program and improved to 42.6% of them post training program. Conclusion: After the implementation of assertiveness training program, there was a highly statistically significant decrease in the total mean score of the aggressive behavior and also, improvement in total scores of assertiveness among preparatory school students. Recommendations: A guidance booklet should be available for students and teachers at school library about the importance of assertiveness and developing students' social skills.