We proposed a liquid immersion cooling technology with natural convection for high power server boards used in data centers with high cooling-efficiency. The cooling performance was evaluated by CFD simulation and actual experiments. As the refrigerants, several non-conductive, thermally and chemically stable fluids were applied, including silicone oil, soybean oil, and perfluorocarbon structured liquids. The CPU temperature in the refrigerant monotonically decreases with the Rayleigh numbers of the refrigerant. The smoother refrigerant is better for cooling the high power CPU. When the upper limit of the junction temperature of the CPU was 98 degrees C (208 degrees F), it was found that 120 W class CPUs can be cooled down. This shows that the technology can be applyed for cooling system with exceeding 10 kW per bath tub. As a result, this proposed technology with natural convection exhibits promising potential for low energy and space-saving board cooling which demonstrates a Power Usage Efficiency (PUE) below 1.04.