Abstract: Geometrical optics/area projection(GO/AP) is a hybrid high-frequency method, which can estimate the corner reflector monostatic radar cross section(RCS). The general process of RCS estimation using GO/AP method is proposed, and the complete RCS analytic expressions of triangular trihedral corner reflectors are derived, which are compared with FEKO simulation results as well as empirical formula of maximum value, and the validity of GO/AP method is confirmed. The GO/AP method is improved at boundary angles of incidence further broadening the adaptive range of the GO/AP method.