A wave coupling theory of linear electro-optic effect is used to investigate the temperature properties of electro-optic modulator made by a uniaxial crystal LiNbO3. The dependence of output intensity on the incident direction and the temperature is characterized by numerical calculations. It is found that along some directions, the output intensity of the modulator is insensitive to the temperature within a range of angles. Further, when the polar angle θ is set to 0, azimuth angle  changes from 0 to π/2 and the temperature T changes from 243 to 343 K, the modulator presents perfect temperature stability. In addition, when  is set to π/8 and θ changes from 0 to 0.0032π, the temperature scarcely has effect on the ratio of output intensity to input intensity. Therefore, the temperature stability can be ensured by adjusting the incident direction of the beam, which can be obtained from the wave coupling theory of linearly electro-optic effect.