In this days, heat pipe heat sink has been widely applied to power controllers for railway substations to remove heat from power semiconductors(diodes or thyristors). The heat pipe heat sink consists of a aluminum heating block for mounting the thyristor, 2~3 heat pipes and large number of aluminum fins. The present study was to get fundamental informations of the structure, design parameters and heat transfer performances of heat pipe heat sink. Series of operational test for a unit with 3 heat pipes were performed and its heat flow circuit was analysed from the experimentally obtained data on wall temperature distribution. Total resistance was ranged 0.02~<TEx>$0.03^{circ}C$</TEX>/W for a power range from 40W to400W. The time to get the steady state was approximately longer than 20 minutes, and overshooting was not occurred during start up operation.