In all countries, banks have been played a major role in financing and providing financial services in the economy by granting loans and accepting deposits . Therefore the bank’s efficiency is one of the most important issues in an economy that interests much attention. In some cases , mergers are common methods to increase the efficiency financial institutions and it is one of the ways of restructuring banking and financial institutions as well.  The purpose of this paper is to investigate economies of scale after taking place of the merger in Iran. For this purpose, by using data sheets, bank merger of Melat Bank and Tejarat Bank that have the most assets between the Iranian banks has been simulated in duration 1382-1391.By using the Tranclog cost function and SUR method, effect of the economies of scale and closing branches were investigated on reducing costs of banks that were merged hypothetically. The results show that the economies of scale is an important reason of reducing the cost after taking place of the merger but closing branches of target bank post merger will not lead to reduction in costs.