As a propellant for hybrid rocket engines using liquid oxidizer and solid fuel and for liquid rocket engines, the use of self-pressurized fluids such as nitrous oxide has become widespread. Since these fluids can be self-pressurized by their high saturated vapor pressure, the propulsion system becomes smaller and simpler. However, this self-pressurization generally forms a gas–liquid two-phase flow by flashing or cavitation. This flow is considered highly unsteady because the temperature and pressure greatly change with the discharge process. In this study, unsteady flow characteristics due to self-pressurization were experimentally obtained by conducting many cold flow tests with carbon dioxide as self-pressurizing fluids. As a result, it was clarified that the fluid temperature dropped about 10–15 K with the pressure drop due to feedline pressure loss during the discharge process. From these experimental results, we estimated the bubble growth and void fraction change that would satisfy the temperature drop. In this paper, the obtained test results and estimated temperature drop are reported.