An important issue that needs to be addressed is to know the relationship between efficiency and NPL/NPF of conventional and Islamic banks. In spite of the numerous studies that have been documented of bank's efficiency, especially for conventional banks, to date, there is no comprehensive evidence which compares the inter-temporal relationships between efficiencies and NPL/NPF of conventional and Islamic banks in OIC countries for the period 1993 - 2007. In the first section of this study, the Data Envelopment Analysis (DEA) approach is used to measure cost, revenue and profit efficiencies of conventional and Islamic banks. The second section of this study is to investigate the determinants of cost, revenue and profit efficiencies. The third section of this study is to investigate the determinants of NPL/NPF of conventional and Islamic banks. The fourth section is to test the inter-temporal relation between bank efficiency (cost and profit), capital and problem loans/financing, as proposed by Berger and DeYoung (1997).