Thermal design of power modules is necessary to miniaturize and ensure reliable operation of a high-density power conversion system. Miniaturization of the power module reduces the heat conduction area and increases thermal interference among the chips in the power module package. They increase the thermal resistance from the junction of the chips to the heatsink. This article designs a multichip power module with low thermal resistance using a high thermal conductive and anisotropic material graphite. The power module substrate that uses graphite as a heat spreader for a multichip power module is designed based on numerical simulation using the finite-element method to reduce the thermal resistance and thermal interference effect of the power devices on the power module substrate. The characteristics of the designed power module substrate are experimentally validated. The developed graphite power module reduces thermal interference by using its thermal anisotropy and achieves at least 8% lower thermal resistance than the conventional copper power module.