Internal audit has an important role in helping a firm to achieve its objective when it is implemented, operated, and managed effectively. Organizations which effectively implemented, operated and managed internal auditing were better able to identify their business risks and system inefficiencies for the sake of taking corrective actions and improvement in the process. The overall objective of the study is to identify the factors determining the effectiveness of internal audit in Ethiopian commercial banks. The study used purposive sampling method and covered the head office internal auditors of Ethiopian Commercial Banks. This study includes both quantitative and qualitative research methods. The quantitative research method used through 171 usable self-administered questionnaires. These questionnaires were constructed in 5-point likert scale, distributed to internal auditors of Ethiopian Commercial Banks, and analyzed using STATA software. The qualitative research method used through personal interview with internal audit directors of Ethiopian commercial banks. According to the results of the study; all five examined independence variable, competence of internal audit, management supports, independence of internal audit, quality of audit work and the relation between internal and external auditors were strong and statistically significant effect on the effectiveness of internal audit regarding on value adding role, enhance department performance, and enhance organizational performance. The researcher mainly recommended; give attention on the development of certified personnel and internal audit work should be assessed externally by an independent reviewer.