This study aims to examine to what extent internal audit effectiveness could be influenced by selected factors namely internal audit scope, internal auditor’s experience and senior management's response in the Omani public sector. Employing questionnaire including four sections with 48 questions for responds of 45 managers and 163 employees in Public Authority for Social Insurance (PASI) in the Sultanate of Oman , descriptive analysis, correlation analysis, and multiple regressions were employed to examine the relationships among the study’s variables. The study found that a significant relationship between internal audit effectiveness and its scope and auditors’ experience in the employees’ sample while they were insignificant in the managers’ sample. Finally, it is found that there is insignificant relationship between internal audit effective ness and senior management's response. Based on t he findings, an awareness has to be maximized on employees to better cooperate with internal audit staff to improve the applications of internal audit standards. Managers are well interested in organization’s activities and performance depending on the internal audit findings and observations. The board of directors has to take in its considerations the reasons of these insignificant results if the reasons are not related to the small size of the managers’ sample. The study’s contribution is to provide evidence regarding the influence of three mentioned factors on the internal audit effectiveness in public sector.