A numerical study on the Trombe wall system, a kind of passive solar systems, has been peformed. The system is modelled as the 2-dimensional steady laminar flow with the natural convection. The PHOENICS code was employed to analyse the performance variation due to the change in the geometrical factor. The mass flow rate and the maximum temperature are changed by the variations in the width of the vents, the width between the window and the wall, and the location of the vents. And there exists the optimal condition to maximize the utilization factor. Further precise analysis has been performed to show the optimal geometry with regard to the above three factors.