The thermal effects in laser crystals have important influences on the properties of laser beams in a laser resonator and an amplifier. They not only change the beam wave fronts, but also affect the polarization states of the beams. Based on the elasto-optic effect of laser crystal, the thermally induced birefringence effect in natural birefringent crystals is investigated. It is showed that the effect of the elasto- optic effect on optical indicatrix in natural birefringent crystals is characterized by elliptical principal axis deflection in the cross section. The deflection angle of the principal axis is less than 0.01°. If a linearly polarized beam passes through a natural birefringent crystal with thermal effects in a laser resonator or an amplifier, the depolarization ratio is less than 1.8 × 10- 8. In this condition, the effect of the thermally induced birefringence effect can be ignored. This characteristic is quite different with that in the optically isotropic crystals.