Background: The incidence of sexual transmitted infections (STIs) and HIV/AIDS is globally higher in young people. This study evaluated the prevalence of sexual reproductive behaviors among undergraduate students of Mashhad, Iran. Methods: The study was conducted on 605 students in twelve non-medical faculties of a great university of Mashhad. A self-administered questionnaire was completed on demographic information, sexual contact in the lifetime and during the last three months, and age of first sex. Kaplan-Meier statistic was used to calculate the mean age of initiation of sex. A p<0.05 was considered statistically significant. Results: After exclusion of individuals over 25 years of age, among 590 students with a mean age of 20.8±1.5 years included in the analysis, 71.4% were female and 85.3% were single. Prevalence of at least one sexual contact in life was 15.1% and 35.3% of single sexually experienced students reported to have sex in the last three months. The lifetime prevalence of sexual relationship in males was significantly higher than females (32.9% vs. 7.6%, p<0.001). The mean age of first sexual experience was 23.7 years with a significant difference between both sexes (p<0.001). In single sexually experienced students, the mean age at first sex was 17.6±3.3 years, 24% started sexual activity at <15 years, 34.3% had at least 3 partners and only 40.6% stated using condom in their last sex. Conclusion: Although very small proportion of females reported premarital sex, a significant minority of male students experienced sexual and risky behaviors. Therefore, the use of educational programs on related issues to reduce the risk of STIs/ HIV among youth including university students seems to be a necessity.