A two-phase flow thermal control system, which utilizes the latent heat of fluid, is paid attention to for the heat removal in the space station and the Space Solar Power System (SSPS). The characteristics are that the system weight is lower than that in single-phase flow and that the temperature can be controlled by changing the pressure of the accumulator. An active thermal control system which can give a maximum 1 kW heat load to an evaporator was built and an experiment on the dynamic characteristics under 1 G were carried out in the extensive quality range with a partial heat load, using alternative freon HCFC-123 as the working fluid. As a result, the effects of the operating quantities such as the heat input and the pressure in the accumulator on the dynamic response are clarified. It is shown that the effect of the operating condition on the time constant of the evaporator is correlated with the quality.