Hydrogen gas evolution from water dispersing nanoparticles under 60Co γ-ray irradiation was investigated under various conditions. Dispersion of TiO2, A12O3, ZnO, ZrO2 and CeO2 nanoparticles with average sizes of 6 nm - 11 µm showed hydrogen yields much higher than that obtained by pure water radiolysis, and the yields seemed to depend on their particle size and shape rather than their chemical species. Hydrogen yield increased with increasing γ-dose but not linearly, which implies the occurrence of a kind of reversing reaction. Some of the nanoparticles supported with Pt, Au and Pd were also examined. The yields were strongly affected by these noble metals, which indicates an important role of the particle surface in the total mechanism of the present hydrogen evolution.