The study investigated the relationship between internal auditing practices and the effectiveness of fraud prevention at Harris International Ltd, a leading company in Kampala, Uganda. The research employed a case study design to conduct an indepth exploration of the organization's internal auditing and fraud prevention mechanisms. Data was collected through structured interviews and document analysis, involving a sample of 50 participants from different departments. The theoretical framework was based on agency theory and fraud triangle theory, which provided insights into the potential conflicts of interest and factors driving fraudulent behaviour within organizations. The findings revealed that the organization had implemented various internal audit measures, such as control environment, control activities, and risk assessment. However, the study identified gaps in the effectiveness of these measures, leading to issues like out-of-budget spending, non-compliance with policies, and low employee productivity. The research provided recommendations to strengthen the internal audit system and enhance fraud prevention at Harris International Ltd.