The main aim of this study was to investigate factors affecting the internal audit effectiveness in the International Non-Governmental Organizations in Ethiopia. The study adopted a cross-sectional descripto-explanatory research design whereby the researcher employed questionnaires and interviews for data collection and also a quantitative and qualitative research approach was used to collect and analyze primary data. Primary data was collected through a survey instrument adapted from different business researches and by using purposive and stratified sampling techniques. The survey data were analyzed through descriptive statistics, correlation and multiple regression analysis. The findings of the study indicated that management support, internal auditor’s independence, internal audit staff competence, and internal audit quality have positive and significant effect on internal audit effectiveness at 95% confidence level. The study primarily recommended the creation of a comprehensive structure to aid in the recruitment of internal audit personnel. Secondly, the study suggested that International Non-Governmental Organizations embrace technology in their internal audit functions. Finally, the audit process should be conducted independently and objectively, with the IA department filled with highly competent individuals who are consistently trained and developed to conduct successful IA.