Optic parameters, such as the probabilities of radiative and non-radiative transition and the cross-relaxation probability between Yb3+ and Ho3+ ions in Yb,Ho:YAG crystal, are calculated on the basis of Judd-Ofelt and Dexter theories. The energy up-conversion process is analyzed by solving the transition rate-equations. The results show that (1) the intensity of the green fluorescence relates to the square of the concentration of the active ions; (2) the intensity increases with the concentration of sensitive ions as well, but the increasing rate goes rather too slow; (3) the efficiency of the energy up-conversion relates with the speed of the energy up-conversion and the quantum efficiency of the transiting from upper level to lower level. Optic parameters, such as the probabilities of radiative and non-radiative transition and the cross-relaxation probability between Yb3 + and Ho3 + ions in Yb, Ho: YAG crystal, are calculated on the basis of Judd-Ofelt and Dexter theories. The energy up- The results show that (1) the intensity of the green fluorescences to the square of the concentration of the active ions; (2) the intensity increases with the concentration of sensitive ions as well, but the increasing rate goes rather rather slow; (3) the efficiency of the energy up-conversion vs. the speed of the energy up-conversion and the quantum efficiency of the transiting from upper level to lower level.