The supercritical water reactor (SCWR), which is one of the generation IV reactor concepts, has particular thermal hydraulics features. If a severe accident happens and pressure and mass flux in a reactor core are rapidly decreased, a film boiling on a fuel cladding tube surface may occur at subcritical conditions. Once the film boiling happens, heat transfer on the cladding tube surface drastically deteriorated and may result in serious damage to the reactor core. The cooling capability during the film boiling depends on the wetting phenomenon, therefore, experiments to clarify wettability phenomenon in subcritical condition are required. One of the experiments to clarify the wettability phenomenon is the capillary action experiment. In the closed system, the water level will elevate due to the injection of the water. The difference in water elevation is due to the capillary force in the different diameter of the pipes. Based on the different water levels with known surface tension, it is possible to quantify the contact angle. The challenge of the experiment is to measure the precise elevation of the water in small diameter metal pipes under high-temperature and high-pressure condition. Therefore, the neutron imaging was applied in this experiment. Neutron imaging is a structure visualization technique. The principle is the neutron flux captured after passing through the object for visualizing the structure of an object. Neutron flux which is captured using a scintillator plate thus can be seen as an image using CCD video camera. Our research group focuses on the radiation induced surface activation (RISA) effect. Significant improvements of surface wettability and boiling heat transfer on oxide film coated-materials by the RISA were confirmed especially under room temperature conditions. In this present research, we evaluate the RISA effect on capillary action in a subcritical condition using the various diameter of the pipe. Neutron imaging was used to visualize the water-gas interface in small diameter stainless steel pipes. The capillary pipes with various inside diameters such as 0.5, 0.8, 1.2, 1.4, and 1.8 mm were used as a test section which was heated up to a temperature of 320 C under a pressure of 21 MPa. The pipes irradiated by ?-ray with an integrated irradiation dose of approximately 500 kGy and non-irradiated pipes with various diameters are installed in parallel and water levels in each pipe were compared to evaluate capillary action differences. o