Fundamental knowledge on surface wettability and boiling heat transfer on metals at sub-critical conditions under radiation is important in thermal-hydraulic design and safety analysis of reactor core in light water reactors including a supercritical water-cooled reactor (SCWR). The radiation induced surface activation (RISA) which enhances wettability and anticorrosive effect on metal surface was first revealed by authors in 1999. In the earlier studies, significant improvements of surface wettability and boiling heat transfer on oxide film coated-materials by the RISA were observed in a room temperature condition. The purpose of this study is to evaluate the effect of oxidized metal and γ-ray irradiation on metal surface wettability in high temperature conditions. A specimen was installed in a pressure vessel. Test section was pressurized at 12 MPa with nitrogen gas and was heated up to temperatures of 20, 150, 200, 250 and 290 centigrade. Two types of material; a stainless-304 and an austenitic stainless steel named PNC1520, which is considered as a potential material of fuel-cladding tube of the SCWR, were used as specimens. The oxide film on specimen was formed in supercritical water at 380 centigrade and 22 MPa. Co-60 γ-ray source was used for irradiation and the cumulative radiation exposure was approximately 600 kGy. The results showed that the difference of oxidization on wettability was insignificant at room temperature before γ-ray irradiation while contact angles on the oxidized specimen decreased at high temperatures. On the other hand, the surface wettability on specimens after γ-ray irradiation was improved regardless of specimen type at relatively lower temperatures less than 250 centigrade, while the changes in contact angle due to radiation was reduced at high temperatures over 290 centigrade. This result suggests oxide film formation on metal surface plays an important role in surface wettability enhancement by the RISA.