Abstract: Based on the physical optics(PO) theory, perfectly electrical conductor(PEC)and leontovich boundary condition(IBC), the high-efficiency formula of the conducted flat and the PEC flat coated with radar absorb materials(RAM) is built up.This formula can also be used on the surface of cylinder and sphere surfaces. Meanwhile, several results are given to demonstrate the validation of this method. Finally, radar cross section (RCS) of an object with super large electrical size computed by this method shows the high efficiency and practicality.