Net interest margin is a significant indicator of the efficiency of the banking financial intermediation. In general, the level of net interest margin is primarily a consequence of result of the level of development and competitiveness of the financial system of country.Therefore, It is important to determine their determinants. In this article, we analyze the determinants of net interest margin of 18 banks in Tunisia between ( 2000…2013). We found that among the internal factors, size, deposits, TLA, CEA, risk have an significant impact  on net interest margin. In external factors, only inflation have a significant impact on net interest margin..