Following the improvement of power converter integration, thermal system design has become an increasingly important issue. The rectangular fin heatsink is widely used in the cooling system of low-power converter because of its reliability and low cost. In this paper, the average convective heat transfer coefficient model of natural convection heatsink is given according Nusselt number experimental correlation. Furthermore, based on the proposed model, the optimal design procedure of heatsink under the natural convection is built. Finally, the effectiveness and accuracy the proposed model and optimal design procedure are verified by the CFD simulation.