An Internal audit is an independent ,objective assurance and consulting activity designed to add value and improve organization operations to accomplish its objectives by bringing a systematic, and disciplined approach to improve and evaluate the effectiveness of risk management, control, and governance processes. The overall objective of this study was to examine factors affecting the effectiveness of internal audit in Ethiopian private commercial banks. The study focused on all Ethiopian private commercial banks in case of Bahir Dar branch. Explanatory research design has been used with primary data by employing quantitative research method and a crosssectional survey design. The quantitative research method was used through 95 usable selfadministered questionnaires. The questionnaire were constructed in to a likert scale form and distributed to internal auditors, accountants and managers and analyzed by using statistical package for social science software. The Ordinary Least Square linear regression model was used to analysis the collected data and examines the association between internal audit effectiveness and seven study’s variables. According to the results of the study competency of internal audit teams and management support were the two major statically significant factors which determining the effectiveness of internal audit in Ethiopian private commercial banks. The researcher recommended in order to improve internal audit effectiveness through competency of internal audit staff the bank should be providing effective training and professional certification program to internal auditors in a timely and planned manner and private commercial banks in Ethiopia should give more attention to those factors which has positive effect on internal audit effectiveness which are competency of internal auditors, independency of audit teams, training of auditors, support from management, approved internal audit charter, relationship between the internal and external auditor which plays a vital role for the effectiveness of an internal audit.