It uses the physical optics method(PO) to analyze the electronmagnetic scattering from electric-large size antenna-radome systems. Under assumptions of locally planar radome surface and locally plane wave characteristic of waves scatter from antennas, formulations of equivalent electric and magnetic currents on radome surface and the induced current on antenna surface are presented. Both monostatic and bistatic radar cross section(RCS) of a aperture anternna, a multilayered radome and the antenna radome system are calculated. Frequency response of the monostatic RCS and the effect of antenna inclination on RCS are also discussed. Numerical examples show efficiency of the PO method and significant effects of radomes on the scattering of antennas.