The aim of this study is to investigate the effect of auditor Industry specialization on the Relationship between Related party transactions (RPTs) and audit fees of listed companies in Tehran Stock Exchange. For this purpose, two hypotheses are developed and data on 65 companies in Tehran Stock Exchange for the period of 2009 to 2015 were analyzed. The regression model was exploited using panel data with a fixed effects approach. The results showed that related party transactions (RPTs) a significant positive association with audit fees. In fact, the result indicated that the auditors increase audit effort (as reflected in audit fees) to address risk associated with Related party transactions. Also, the findings indicated that auditor industry specialization induce moderating relationships between related party transactions and audit fees. In other words, positive association between related party transactions (RPTs) and audit fees is weaker for firms audited by industry specialist auditors and firms audited by non-specialist auditors.