This paper presents the direct liquid-cooled power module with the circular pin fin which is the inverter parallel cooling system for high output EV/FCEV. The direct cooling system of a conventional inverter is designed to supply coolant along the direction in which the heating element such as Si-chip is disposed and discharge coolant to the opposite side. In case of the inverter, the higher the output is, the larger temperature difference between inlet and outlet becomes due to the heat exchange of the heat generation element, so that temperature difference depends on the position of Si-chip. Since lifetime is judged on the basis of maximum temperature of Si-chip, the inverter itself must be replaced or discarded due to durability of the inverter even though Si-chip can drive further. The simple way to solve this problem is to increase cooling flow rate, but this leads to excessive increase in pressure loss due to circular pin fin. Purpose of this study is the concept of parallel cooling system which improves junction temperature of Si-chip and pressure loss in cooling channel at once without increasing flow rate when configuration of the circular in-fin is fixed. Numbers of geometries of parallel cooling channel have been modeled for optimization. The decrease in rising temperature of the best geometry achieved about 2.4% which is the same result as 15.6% increase of flow rate compared to conventional design at 8 L/min on 450 V, 8 kHz switching frequency. However, the pressure loss of the best geometry was reduced by 36%.