Background: The current number of sero-positive HIV patients in Ethiopia is about 700,000 with overall estimated prevalence of 1.5%. The introduction of combination ART including protease inhibitors has resulted in striking reductions in HIV-related mortality. Numerous reports have documented that the key to the success of the new HAART is the ability and willingness of HIV-positive individuals to adhere to complex ART regimens, and at least 95% adherence is required for ART regimens to be fully effective. Objectives: The main aim of this study was to assess level of adherence to ART and associated factors among HIV sero-positive adult on HAART. Methods: An institutional based cross-sectional study was conducted on 377 participants selected by systematic sampling technique. A pre-tasted structured questionnaire was used by employing interview to collect necessary data. The collected data were analyzed using SPSS 16.0 version. Significance level was set at 95% CI and p-value of <0.05. Results: The adherence rate of the study participants was 88.6%. Monthly family income [(AOR 0.3, 95%CI 0.13, 0.69)], delayed in taking ART drugs AIDS [(AOR 0.6, 95% CI 0.16,0.88)], fitness of daily treatment schedule [(AOR 9.7, 95% CI 4.6,28)] and consistently taking of ART drugs [(AOR 5.7,95%CI, 2.6,25.3)] were significantly associated with ART adherence. Conclusion: The ART adherence level in this study was low. Delay in taking ART drug AIDs, monthly family income, fitness of daily treatment schedule and consistent taking of ART drugs were significantly associated with ART adherence. Measures have to be taken to address these problems.