Abstract: This study aims to present a picture of how independent auditor rely on internal auditor activity to improve the internal audit function through belief functions in Financial Year (FY) of 1392 in the Sepah Bank. The results of the investigation revealed that impartiality is one of the important factors in assessing the IA function strength. Even when exist high levels of professional competence and work performance of internal auditor when there is not available appropriate information about the impartiality of the internal audit function, the strength of IA function is placed in lower position.