The absorption cross-section of Ce3+ ions in the Ce3+,Er3+∶YAG crystal has been measured.The laser threshold energy (or power) and laser slope efficiency are calculated using a formula deduced from rate equations on the transitions of energy levels.It is shown that the effect of Ce3+ ions in Ce3+,Er3+∶YAG on properties of the laser at ～1.6μm wavelength depends upon the active ion concentration and the length of the crystal rod.The effect of Ce3+ ion is to reduce the laser threshold energy and increase the slope efficiency,when the crystal rod is shorter or the concentration of Er3+ ion is lower.But when the crystal rod is longer or the concentration of Er3+ ion is higher,the effect of Ce3+ ions is on the contrary.