We investigated the effect of the gamma-ray irradiation on the emission property of the Eu ion doped YVO4. We clearly observed that on exposure to the gamma-ray, sharp emission peaks originating from the Eu3+ ions were suppressed dramatically. Instead, a broad emission feature near 470 nm, which was attributed to the Eu2+ ions, emerged. The quantitative analysis on the emission spectra suggest that the valence state of the Eu ions in our samples was changed from 3 + to 2 + by the gamma-ray irradiation. The conversion of the valence state of the Eu ions was closely related to the formation of the oxygen vacancies induced by the gamma-ray irradiation.