Background: HIV/AIDS is a major public health problem in Ethiopia. University students are often a young and sexually active group that is at risk of acquiring and transmitting HIV. We assessed risky HIV sexual behaviors and utilization of voluntary counseling and testing services among undergraduate students at Addis Ababa Science and Technology University, Ethiopia. Methods: A cross-sectional study was conducted between May and June, 2013. Standardized semi-structured self-administered questionnaire was used to collect data. Simple random sampling technique was use to select departments from each school. All students in the selected departments were the study participants. Data were entered into EPI-Info and analyzed using SPPS statistical packages. P-value <0.05 was considered as statistically significant. Results: Of the total 602 students selected, an overall response rate of 557 (92.6%) were registered. Among the participants 361 (60%) were males. The student ages' were ranged from 17 up to 25 years with mean age of 20.3 +/- 1.6. Around 385 (64%) of them were in the age group of 17 up to 20 years. Among the study participants, 161 (26.8%) had sexual contact and the mean age of first sexual encounter was 17.4 (SD = 2.3) years. About 443 (76%) of students knew that condoms can prevent Sexually Transmitted Infections (STIs). Among sexually active students, 74 (46%) had not used condom during first time sex. Among those responded, 488 (83.4%) had heard information about VCT; however, 52% had not ever used VCT service. The overall mean score of knowledge and attitude of students towards risk perception on HIV was around 66% and 57%, respectively. Students who enrolled in health science departments had almost three time more knowledge [ AOR(95% CI) = 2.83 (1.67, 4.80)] and two and half times more favorable [AOR (95% CI) = 2.55 (1. 60, 4.06)] attitudes towards HIV risk reduction strategies than students in non-health related departments. Conclusions: Some students were engaged in risky sexual behaviour even though they had heard about HIV/AIDS. The perception of risk for acquisition of HIV infection and utilization of VCT were low. HIV prevention and control strategies including education in the areas of HIV/AIDS as part of university programs curriculum, specially non-health students, and strengthening health institutions to provide youth-friendly VCT services for HIV with "know your HIV status" campaigns are strongly recommended.