In unannealing Yb: YAG crystals, the absorption peak positions of the color center were located at the wavelengths of 375 nm and 625 nm, and its concentration was increased with doping concentration of Yb 2O 3. The formation mechanism of the color centers was discussed. Large quantity of color centers were produced in Yb: YAG crystal under γ irradiation. In order to improve the fluorescence lifetime of Yb3+ , the color centers should be eliminated.