Gas/liquid phase changes produce large volume changes in working fluids. These volume changes are used as the driving power sources in actuators such as micro-pumps and valves. Most of these actuators are utilized in ordinary temperature environments. However, the temperature range in which the phase change actuator can operate depends on the characteristics of the working fluid. We hypothesized that proper selection of the working fluid and the structure of the actuator can enable such actuators to be applied not only in ordinary environments but also in high temperature environments. Consequently, in this paper, we discuss the design and fabrication of a new gas/liquid phase change actuator for use in high temperature environments. Our proposed actuator consists of a bellow body, spring, heater, and working fluid. We used the Inconel super alloy, which is highly heat and corrosion resistant, for the bellow and moving parts of the actuator. For the working fluid, we prepared triethylene glycol, which has a boiling point of 287.3 °C and very low vapor pressure at ordinary temperature. As a result, our proposed actuator can be utilized in high temperature environments up to 300.0 °C. The results of several experiments conducted confirm that our proposed actuator generates 1.67 mm maximum displacement in a 300.0 °C atmospheric environment. In addition, we confirmed that the operation of the actuator is stable in that environment. Our results confirm that a gas/liquid phase change actuator can be used in high temperature environments.