Abstract: Modified Cassegrainian antenna with polarization rotator has been widely used in the airborne radar systems because of its electrical and structural advantages. Up to now, however, the scattering aspect of this antenna has not been systematically investigated theroe-tically and experimentally. In this paper, the radar cross-section (RCS) of this antenna under the incidence of EM waves of various frequencies and polarizations is evaluated based on antenna scattering theory. The numerical results show that this kind of antenna contributes a substantial RCS in the nose-on region, and it will be quite a difficult task to reduce its RCS contribution without- degradation of antenna performances.