Two-phase flow loop cooling system is required for increasing heat transfer rate, heat transport distance, and cooling heat flux in thermal control of space structures. The understandings on thermo-fluid dynamics of gas-liquid two-phase flows with phase change under microgravity is necessary for the system design. To clarify the phenomena and verify the operation of two-phase flow loops under microgravity condition, two-phase flow loop experiments were conducted as a JAXA project named TPF experiment onboard the International Space Station (ISS). The loop was a pump-driven loop and the pressure was maintained by an accumulator. Perfluorohexane was used as the working fluid because of low latent heat of vaporization and moderate boiling point under the restriction of power input and for the safe operation. The loop had two kinds of heating section, such as a copper heating tube and a transparent glass heating tube, and a condenser. Flow observation sections made of transparent polycarbonate resin were installed just downstream of each heating tube in order to observe and measure the interface structure of boiling two-phase flow in detail. In this report, the design concept and system characteristics of the experimental loop and the results of the gas-liquid two-phase flow measurements in the flow observation section are presented.