Effect of surface wettability on boiling heat transfer was studied by applying photo-catalysis effect and radiation induced surface activation. The experimental apparatus consisted of a copper heater block, rectangular container, and thermostat bath. Two copper heater blocks were fabricated: one to examine the radiation-induced surface activation and the other, whose heat transfer surface was coated by a TiO film, to examine the photocathode reaction under ultraviolet radiation. These blocks were irradiated by ultraviolet light or γ-rays to change the surface wettability. The boiling heat transfer under subcooling conditions was measured before and after irradiation to study the effect of the wettability. The experimental results show that the nucleate boiling curves for the irradiated surface are shifted to the higher wall superheat side owing to a decrease in the active nucleation sites. In contrast, the boiling curve after irradiation moves to the lower wall superheat side in the micro-bubble emission boiling regions. 2