Power semiconductors such as a GTO thyristor have recently come to be used in inverters and converters for electric train motors, other rotary machines and so on. Such power semiconductors require cooling units with a high cooling capacity and safety considerations such as electrically isolation from live parts. To meet these requirements, the authors started developing an electrically insulated heat pipe type cooling system for thyristor. The electrically insulated heat pipe the authors have in mind is characterized by an insulator pipe (alumina ceramics) isolating the condensing section from the evaporating section electrically and an electrically nonconducting fluid in the pipe. An additional advantage of the heat pipe type cooling system is that it uses significantly less FREON than that of traditional tank type. The authors conducted various tests on the electrically insulated heat pipe to confirm its reliability so that it can be put in practical use on an electric train. This report contains results of representative dielectric, individual performance, unit performance, and vibration tests.