The concern about the effects of long-term, auditor-client relationships on auditor independence and audit quality has led to enact a series of rules that limited the relationships.While the question of mandatory auditor rotation has received continued attention from regulators as means of strengthening independence and improving audit quality, but support for mandating auditor rotation has been mixed.Becouse of regulators’ concern about the impacts of auditor tenure, this study examines the association between auditor tenure and an important feature of accounting, namely conservatism. In this study, conservatism is operationalized by using of Basu’ model that based on quicker recognition in earnings of bad news about expected future cash flows. By using of multiple linear regression for Tehran Stock Exchange (TSE) listed firms during 1381-1385, we document a positive and significant relation between auditor tenure and conservatism. In addition, by partitioning the sample based on the length of auditor tenure into three groups, primary results confirmed and it is observed that conservatism increases with auditor tenure. Collectively, the results imply lower conservatism for short auditor tenure. Thus, mandating auditor rotation may have an adverse affect on the conservatism in reported earnings.