Abstract: The optic axial angle of biaxial crystal is calculated by coordinate transforming the index ellipsoid equation X^2/n1^2＋y^2/n2^2＋z^2/n3^2=1from the principal dielectric coordinate system to another new coordinate system.This new coordinate system Ox′y′z′is gained by rotating the coordinate system Oxyz. It is confirmed that z′axis is optical axis if the intersection of the new index ellipsoid and x′y′ plane is a circularity.

Key words: biaxial crystal, optic axial angle, index ellipsoid, coordinate transformation