Abstract: The theorem of ellipse in perspective drawing is introduced in this paper. Found a kind of internal relation between the center projection and the parallel projection, under the center projection, the circle is changed into ellipse, but it can’t decide the relation between the conjugate axis or major axis and minor axis of ellipse, which can be solved by this theorem. Theoretically, it provides a new evidence for computer to use program of ellipse avalilable to draw ellipse in perspective drawing.