In this paper, we’ll try to study the impact of ownership structure (state owned/private) of banks in Tunisia on its level of cost efficiency. While, we have proposed an assessment of the cost efficiency of Tunisian banks during the period 1999-2009 on a sample of 17 universal banks using a stochastic frontier model (SFA). We introduced in the translog cost function of the external variables related to the environment in which banks operate in order to know whether these variables affect the cost efficiency scores of banks or not. Overall, our results demonstrate that environmental variables contribute significantly and positively to the difference cost-efficiency scores between Tunisian banks. Also, our results show that private banks in Tunisia are more efficient than public banks, but there are differences among private banks. Privatized banks with majority foreign ownership are the most efficient while those with domestic ownership are the least.