This article experimentally investigates the thermal performance of the heat pipe cooling system with the thermal resistance model. Evaporator and condenser, which are the two main devices, connect to each other to form a closed system. The liquid water absorbs heat from heat source and evaporates in the evaporator. The evaporating fluid moves toward the condenser, and then condenses in the condenser. The experimental parameters are different evaporation surfaces, fill ratios of working fluid and input heating powers. The result shows that the evaporation resistance and the condensation resistance both grow with increasing heating power and decreasing fill ratio. Flooding is found at the fill ratio of 20% with the evaporation surface noted Etched Surface 2 when heating power is above 120 W. Flooding phenomenon is caused by the opposite flow direction of vapor and liquid in a closed two-phase system. According to the result, the lowest total thermal resistance is 0.65 degrees C/W by the evaporation surface noted Etched Surface 2 at 30% fill ratio. (C) 2008 Elsevier Ltd. All rights reserved.