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The main purpose of this paper is to examine the effect of merger, concentration and credit risk on the efficiency of Iranian Banking industry. To measure the efficiency of Iranian banking system, we have used the data of commercial & specialized bank's balance sheets during 2001-2007, and a parametric approach to estimate two empirical models. To estimate efficiency measures and determining main factors affecting the measures, we have used a Logarithmic - Linear form of a random Translog cost function. The results of the first estimated efficiency model show that the average efficiency measure of banking system in Iran is 54% and that the merger of the more inefficient banks within the efficient bank will cause the average efficiency measure rise to 70% The results of the second model - assessing the effecting factors on efficiency- show that the efficiency of banks has an inverse relationship with the concentration (competition in banking industry), and a direct relationship with the IT index (e-banking activity) and the facilities to assets and capital to assets ratios (as the indices of the credit risk).