Adolescents are vulnerable to starting smoking and becoming addicted to nicotine. Aim: This study aimed to assess awareness of adolescent students about the harmful effects of smoking at Al-Qaswa schools in Al-Madinah Al-Munawara. Research design: A descriptive design was used in this study. Setting: This study conducted at preparatory and secondary school in Al-Madinah Al-Munawara, Saudia Arabia. Sample: Random sample used to choose 200 adolescent students who aged 13 to 19 years, and accept to participate in the study. Tool of data collection: One tool included five parts, 1st part: Demographic characteristic, 2nd part: Assess prevalence of smoking among adolescent students, 3rd part: Knowledge of adolescent students about smoking, 4th part: Reported practice of adolescent students with prevention from smoking, and 5th part: Assess harmful effect of smoking for adolescent students. Results: 60.0% of the studied students had poor knowledge, and 30.0 % of them had good knowledge. While, 65 % of studied students had unsatisfactory reported practices and 35 % of them had satisfactory reported practice. Conclusion: Less two third of the studied students had poor total knowledge about harmful effects of smoking, and more than two third of them had unsatisfactory total reported practices regarding prevention of smoking. There is statistically significant relation between students' demographic data and their knowledge and reported practices regarding harmful effects of smoking and prevention of smoking.