In this work, we calculate drop in antenna gain, aperture phase distribution, and antenna pointing shift of KVN(Korean VLBI Network) 21m shaped Cassegrain antenna due to misalignments of antenna optics using ray-tracing method. The misalignments we considered are axial displacement of feed, axial displacement of sub-reflector, lateral displacement of feed, lateral displacement of sub-reflector, and sub-reflector tilt. Calculations are performed not only when these misalignments exist separately, but also when they exist at the same time. Although ray-tracing method is based on geometric optics which does not consider electromagnetic effects, we expect that this work enables us to align antenna optics which give the maximum gain.