The purpose of this paper is to systematically review the literature on the influential factors of internal audit effectiveness and articulate these factors in a conceptual model. A systematic literature review (SLR) is conducted to identify the influential factors of internal audit effectiveness; relevant studies are reviewed between the period January 1999 and March 2022 through a lens focused on the key factors of internal audit effectiveness. In addition, our review took into consideration what is mentioned in The International Professional Practices Framework for Internal Auditing (IPPF). Five factors of internal audit effectiveness and their dimensions are identified and comprised into a conceptual model, these factors are internal audit organizational characteristics, internal audit relationships, internal audit processes, internal audit resources, and internal audit coordination with other assurance providers. This paper provides internal audit practitioners, audit committees, and senior management in organizations with a broad understanding and comprehensive overview of the key factors that should be considered to make their internal audit functions more effective. This paper proposes a conceptual model that provides a holistic view of the influential factors of internal audit effectiveness and clearly identifies the dimensions of the factors. Additionally, it provides an opportunity for future research to test the model and build on it as well.