The aim of this study was to ascertain the role of internal auditing in organizational effectiveness focusing on Diamond Bank. The specific objectives are; to determine the role of internal auditing in promoting organizational performance of Diamond Bank, to examine the effect of internal auditing in the internal control systems of Diamond Bank,to ascertain the effect of auditors’ independence on the efficiency of internal auditors and to determine the effectiveness of internal auditing in controlling fraud in Diamond Bank. The study used simple survey. The population of study was management and staff of Diamond Bank Plc with a total population of 79 staff while the simple size was 72. Table and percentage were used for analysis while the hypotheses were tested with chi square. The study found that internal auditing improves the credibility of banks, internal auditing has aided in managerial decision making in Diamond Bank and internal auditing has improved the profitability of Diamond Bank. The study recommends that banks should endeavour to keep to professional codes of conduct and ethical considerations as it enhances the activities of internal auditor and in so doing eliminate cases of fraud in the banking sector. Further, there should be audit education among the stakeholders of diverse organizations.