Abstarct: Achievement of benefits such as better control of the power transfer capability, increasing power and miniaturization of the power electronics is not possible without the development of the thermal management techniques included the interior component design and the proper cooling system design. In this thesis, 3-D simulation of heat transfer in a power electronic device and its cooling system are performed. The device is a high voltage three-phase inverter manufactured by Semikron Company. Limitation factor of designing heat transfer system is maximum temperature of the inverter’s chips, IGBTs. Maximum temperature of IGBTs should be below 125 ᵒC in order to avoidance of both the thermal and the mechanical failures. Cooling system is a forced-air plate-fin heat sink. One of the primary objectives is the reduction of the maximum temperature by designing layout of chips. Also, the heatsink geometry design is accomplished with the consideration of the maximum temperature and tradeoff between both the usage material volume and the heatsink efficiency. The geometries are the number of fins, the fin height, the fin thickness and the baxse thickness of the heatsink. In the design process of the cooled plates, size of the channels, coolant flow path, velocity of coolant, and temperature of coolant with consideration of both the operating temperature condition of the device and the convective heat transfer coefficient. The power dissipation of the chips is estimated by using datasheet information and output waveforms obtained from simulation in MATLAB. A thermal model of the inverter and its cooling system are simulated by using finite-element method (FEM). The accuracy of the thermal model and power dissipation estimation are verified by Semisel software. In this paper, the results of layout design of heat sources show that the maximum temperature is significantly reduced by designing precise layout. Also, outstanding result of heatsink geometry design is that only good design increases the efficiency remarkably with no material volume variation. Investigation of the water temperature on the thermal operating system shows that the cold plate can be used for power inverter cooling with high temperature water.