There has been most interested in the renewable energy not only ballooning oil prices but also Fukusima Daiichi nuclear accident. Photovoltaic energy of all the renewable energy is highly demanded because it is used convenient if only sunlight is available. Photovoltaic Inverters or PV Inverters utilizing a photovoltaic system control the generated power of the maximize solar energy collected from photovoltaic panels. The basic function of PV inverters is to convert the direct current(DC) power coming from a photovoltaic panel into the alternating current(AC). Recently, almost the research for PV inverters has been conducted to reduce the size and weight of them and to improve the power density and the electric power conversion efficiency of the system. So to ensure PV’s competitive price in the market, it is specifically significant to find optimum heat sink designed to cool power semiconductor. In this research, we considered geometrical parameters of heat sink for the selecting proper size, the position of power semiconductor, and the guide of flow in order to find the condition of the safe operation. Also, thermal-fluid analysis was conducted a study on the PV inverter for 4kW class that are being developed in our company and used the ANSYS ICEPAK specialized in CFD simulation of power electronics equipment. And then we found out the temperature of the power switching chips, the distribution of surface temperature on heat sink and the flow condition between fins.