A numerical study of mixed convection flow and heat transfer inside a cavity with a obstacle is performed.Given a fixed location of the obstacle,the size of the barrier is varied along the bottom of the tube in order to determine the best configuration of the system corresponding to maximum heat transfer rate and minimum pressure drop.The governing equations were solved iteratively by the finite volume-method using SIMPLE algorithm.The results are presented in the conventional form:contours of streamlines and isotherms and the average Nusselt number.