A numerical study has been performed to investigate mixed convection flow in a vented square cavity with circular cooling obstacle. The governing equations such as two dimensional Navier-Stokes, continuity, and energy balance equations have been solved using a finite volume discretization method with SIMPLE algorithm. The effect of the Richardson number, outlet port location and volume fraction of nanoparticles were studied. The outlet port is varied from top to bottom in order to find the maximum heat transfer rate. The results indicated that by increasing the volume fraction of nanoparticles and reducing Richardson number, the heat transfer rate is enhanced. Moreover, it is found that the best configuration of the system for better heat transfer is obtained when placing the outlet port on the bottom location of the vented cavity.