As the performance of the advanced electric systems increases, the packaging densities and power requirements will also increase. The reliability of these components will depend on the ability of the packaging system to transport heat away from the device. In this paper, a liquid-cooled coldplate for the inverter of hybrid electric vehicle was designed by using Computational Fluid Dynamics (CFD) technique. The main features of inverter packaging include power module, capacitors, busbar, gate driver, gate power supply, coldplate, sensors, & controllers. How to effectively dissipate the heat from power module to the coldplate is the focus of this study. The 3-phase full bridge power module consists of 12 IGBTs and 12 diodes. The silicon dies of IGBT or diode were soldered to the direct-bonded ceramic (DBC) A1N substrate, and to the copper base plate. Then the whole module was mounted mechanically onto an aluminum coldplate using thermal grease at the interface. The maximum allowable die junction temperature is 125°C. The commercial CFD code, FLUENT, was used here to study the flow field and heat transfer of the coldplate. In order to have confidence in the CFD prediction, the temperature distribution of an inverter assembly was obtained from FLUENT and then verified with the measurement from an infrared camera. Several design options on the coldplate, i.e., diameter & height of fins and shape & pattern of fin arrays, were examined. The effects of coolant flow rate and coolant type on the performance of coldplate were also studied. The overall thermal resistance and pressure drop of the coldplate were used to compare the efficiency of a series of coldplate design. Based on the CFD results, the effect of coldplate pin fins design on the thermal resistance is small. However, the pressure drop of the coldplate is quite sensitive to the design of pin fins. It is also noted that the fin height of coldplate can be reduced by 10% without degrading the performance of coldplate.