Abstract: The relation of ray axis and optical axis is discussed by using the optical wave camber of biaxial crystal. The mathematical expression of included angle between ray axis and optical axis is also given. The result shows that the included angle is much less than 1° in two biaxial crystals (topaz and cordierite). This is important for the design of the optical devices. It indicates that the direction of ray axis and optical axis is identical by discussing the uniaxial crystal at last.