This paper describes a study on 10 kW-class Loop Heat Pipe (LHP) which is a two-phase heat transfer device. In this study, the 10 kW-class LHP was designed, fabricated, and tested. Condenser was cooled under stagnant water condition and forced water convection, and we evaluated steady state temperature. It was experimentally confirmed that this LHP can transport 9.5 kW heat in the steady state under stagnant water condition, a temperature of heater blocks being within 300°C. Then the thermal resistance at 9.5 kW was 0.006 °C/W. Finally, the steady experimental results were compared with calculation results and the maximum absolute error of heater blocks temperature was 22°C.