YAG:Er laser crystals were grown by Czochralski technique with Er

3+

ion concentrations of 1.635×1021/cm

3

1.156×1021/cm

3

0.044×10

21

/cm

3

and 0.029×10

21

/cm

3

respectively. The spectral properties of YAG:Er

3+

crystals were studied and the absorption cross sections were obtained. It is noted that YAG:Er

3+

laser is optically pumped because there is absorption exsting in ultraviolet

visible and near infrared ranges. The energy gaps between some pairs of Er

3+

levels in the YAG:Er

3+

coincide each other

which can cause the cross relaxation process.By analysing the spectra of YAG:Er

3+

crystal

the actions of radiative transition from

4

S

3/2

、

4

I

9/2

、

4

I

11/2

and

4

I

13/2

to

4

I

15/2

levels are discussed.The cross relaxation process between certain pairs of Er

3+

levels in YAG:Er

3+

is a nonradiative process under multipole interaction

in which dipole-dipole interactions are considered to play a dominant role. The fluorescence of

4

I

9/2

and

4

S

3/2

levels is quenched with the increase of Er

3+

concentration

and that of

4

I

11/2

level presents an increase. For

4

S

3/2

、

4

I

9/2

和

4

I

13/2

there is an optimum concentration.