Abstract:Background: Early onset of sexual intercourse associate with increased lifetime prevalence of sexual partners that increase the risk of exposure to sexually transmitted infections and unintended pregnancy. Alcohol consumption, cigarette smoking, or use of illicit drugs by youths associate with increased risks of sexual intercourse, multiple sexual partners and lower rates of condom use. Therefore, this study aimed to determine the proportion of sexual initiation and associated factors among Addis Ababa university undergraduate students. Methods: A cross-sectional institution based study design was conducted from November to December 2013 at Addis Ababa university undergraduate students. Multistage sampling technique was used to draw 1012 sample students. Data were collected using self-administered questionnaire method supplemented by focus group discussion. Bivariate and multiple logistic regression statistical analyses were employed to examine the relationship between sexual initiation and selected exposure factors. Statistical significant association was determined using 95% confidence interval of adjusted odd ratio and p value less than 0.05. Results: Thirty nine percent of the participants had sexual intercourse at the time of the survey, of which 129 (34.7%) were females. The mean age at first sexual intercourse was 17.6(±1.9). Majority 262 (27.4%) of sexually active students had their first sexual intercourse before the age of 18 years old. Being male [AOR=1.4; 95% CI: 1.1, 2.0], alcohol drinking [AOR=2.8; 95% CI: 2.0, 3.9], khat chewing [AOR=2.0; 95% CI: 1.4, 2.9] and watching pornographic movies [AOR=2.9; 95%CI: 1.9, 4.4] significantly associated with sexual initiation. Conclusion: There is high level of sexual initiation among the study participants. Significant proportion of students had multiple sexual partner and sexual practice without condom with non-regular sexual partners. The university and local health bodies should work together to address the identified risky sexual practices with particular focus on behavior change communication.