Abstract: The upconversion luminescence of Yb3+-doped yttrium aluminum garnets under 940 nm excitation at room teperature was studied, and it was ascribed to Yb3+ cooperative luminescence and the presence of rare earth impurity ions. X-ray excited luminescence features of the crystal were also measured and the emission bands correspond to the transition from charge transfer state to the ground and excited state of Yb3+ ion. The emission spectra of Cr,Yb:YAG crystal were studied and the Cr4+ emission band was observe with its peak at 1.35 μm wavelength range from 1.2 to 1.6 μm. The probability of Cr4+ laser output was discussed. 