Through the coefficient of thermal expansion tensor, the formula of the omnidirectional thermal expansion coefficient of the optic calcite crystal (OCC) is deduced, and the zero-expansion direction of the OCC is determined. The expression of the omnidirectional birefringence versus temperature is derived, and the relationship between the omnidirectional optical path difference and temperature is also provided. These results show that the zero-expansion of the OCC occurs at the azimuth angle of 65.35° or 114.65°. The effect of temperature on the optical path difference is subject to the azimuth angle and along the direction perpendicular to the optical axis, this effect is the most.