Background: Early sexual initiation increases the risk of HIV infection and other sexually transmitted diseases. The majority of students in higher Institutions are single, young adults who easily fall prey to excitement coupled with the liberal nature of campus life that predisposes them to high risk sexual behaviour. The aimed of study was to determine the prevalence of early sexual initiation and associated factors among Ambo University undergraduate students in Ambo, Ethiopia. Result: The prevalence of early sexual initiation among Ambo University undergraduate regular students were determined, which showed that 67(20.4%) were initiated early sexual practice with 95%CI of (16.2 to 25.2%) with independent predictors of religion (Christian, AOR=14.66, 95%CI (1.77, 121.51) residence (urban) AOR= 3.06, 95%CI (1.33, 7.02) and classmate friend AOR=21.83, 95%CI (2.65, 179.84). Methods: A cross-sectional facility based study design was conducted from December to January 2015 at Ambo university undergraduate students. Simple random sampling technique was used to draw 392 sampled students. The study participants were selected by using simple random sampling computer generated technique by their ID number. A pre- tested structured questionnaire was used to collect data. Both bivariate and multivariate logistic regressions were used to identify associated factors and the result was presented using table, and narrative. Conclusion and recommendation: The prevalence of early sexual initiation and associated factors among Ambo University undergraduate students were determined which revealed that 20.4% with independent predictors of religion (Christian), residence (urban) and classmate friend. Therefore, these factors would be better to considered during development youths reproductive health programs by police makers in collaboration with others responsible bodies.