Water treeing occurs when ac voltage is applied to electrical insulation system such as those in power cables and power apparatus. From a practical viewpoint, research on water treeing has been mostly performed under high-frequency ac voltage application. However, power cables and power apparatuses are also used under dc voltage application owing to progress in inverter technology. The inverter drive of a rotating electrical machine has spread and expanded for global warming prevention and maintenance improvement. An underwater motor is used in the water. In an underwater motor driven by the voltage of an inverter, it is necessary to consider the water treeing of motor insulation and connected cable insulation with a drive board. Thus far, the water electrode method that utilizes an ac voltage of high frequency has been used in conducting the accelerating experimental method of water treeing for a long time. On the other hand, an inverter with surge voltage generates an equivalent ac frequency by the application of a multifrequency pulse voltage similar to a pulse width modulation (PWM) control wave. Therefore, there is a possibility of committing a mistake in real-machine reliability evaluation because the initiation and propagation mechanisms are different from those in the case of a real-machine even if we can reproduce water treeing by increasing the basic frequency. In this paper, we investigated the new water electrode method and the temperature effects on water tree initiation and propagation. Furthermore, we compared the experimental results between 500Hz and 50Hz by using the new water electrode method from the view point of temperature.