The design parameter of the reactor vessel insulation for the Korea Standard Power Plant has been studied numerically. The heat loss from the reactor vessel through the insulation is analysed by using the computational fluid dynamics code, FLUENT. Parametric study has been performed on the air gap width between the reactor vessel wall and the inner surface of the insulation, and on the insulation thickness. Also evaluated is the performance degradation due to the chimney effect caused by gaps between the panels during the installation of the insulation system. From the analysis results, the optimal air gap width and the optimal insulation thickness are obtained.