Abstract Background and purpose: Priority health-risk behaviors, often are established during childhood and adolescence, extend into adulthood, and are interrelated and preventable. This study was conducted to determine and compare the prevalence of risky behaviors on both sexes of freshman students enrolled in Tehran University, Iran. Materials and Methods: This study was a descriptive-analytical type of cross-sectional survey which has used stratified sampling to select 432 students during 2011-2012. A questionnaire including, 14 demographic questions and 38 questions about risky behaviors such as unintentional intentional injuries, smoking habits, alcohol and drug use, sexual behaviors, nutritional habits, and physical activities was used as the instrument of the study. Attending student’s club and passing medical examination, each student completed the self- reported questionnaire. Results: The mean age of participants was 23/2 ± 5/1, the majority of them were single (90.5%), 80.6% were unemployed, and 60.2% were from other cities. The prevalence of smoking cigarette (P < 0.001), using hookah (P < 0.001), carrying a cold weapon (P = 0.049), and driving without license (P < 0.001) were more in boys than girls while eating fruit (P < 0.001), vegetables (P = 0.049), and meat (P = 0.041) were more in girls. There were no significant differences in other risk behaviors (P > 0.05). Conclusion: Some health risk behaviors in boys were more than girls, and there is a possibility of increasing these high-risk behaviors in the university environment. Thus, keeping students under surveillance and adopting preventive actions play a crucial role, and comprehensive training plans to promote health behavior should be designed and implemented.