In this paper, the authors deal with the effects of γ-ray radiation on crude oils with special reference to the origin of petroleum. The several Japanese crude oils and the mixtures of these crude with pulverized sedimentary rocks and water were irradiated by γ-ray. After irradiation, the amounts and compositions of produced gases, the liquidchromatographic compositions of the heavier components in crude oils and the contents of porphyrins were determined. The experimental results indicate that the gases, constituted by hydrogen, carbon monooxide, carbon dioxide and gaseous hydrocarbons (methane. ethylene, ethane, propylene, propane, iso-butane, n-butane, iso-pentane and n-pentane) are produced, the heavier components in orude oils are polarized and the porphyrins are decreased. These facts may suggest that γ-ray irradiation is considerably effective on the conversion of oil.