The sensitive effect of cerium to erbium ions has been analyzed by solving the rate quations of level transition in Ce, Er: YAG and Er: YAG crystals. It is shown that this effect increases the laser intensity at wavelengths both 1.66 and 2.94 μm, but only changes the relation between the population inversion of 4I 13/2 and 4I 15/2 levels and the concentration of Er 3+ ions.