One of the major problems of the Iranian banking industry is non-performing loans and facilities which are not collected nowadays. Most economists believe that the growth of nonperforming loans is a sign of non-optimal allocation of banking resources showing that the banking system is managed publicly and the emphasis on expanding share of private banks in the banking industry. Thus, with transferring state banks to the private sector in recent years, as well as the establishment of new private banks, it is necessary to investigate the effect of development of private banks on non-performing loans of the banking system in Iran. For this reason, the present paper attempts to test and analyze the effect of development of share of private banks in the banking system on nonperforming loans of all banking system by using seasonal data of 2001 to 2011 and by applying Auto Regressive Distributed Lag (ARDL) model. The results showed that there is a significant long-run relationship between development of share of private banks in the banking system and the whole nonperforming loans in the banking system. The research also emphasis that this correlation in Long-term will be negative.