The finite-element method(FEM) is a popular technique for analyzing electromagnetic scattering, radiation, and penetration. Far-field radiation pattern of circular waveguide bevel cut and fields in the waveguide are analysed with three-dimensional FEM. Results of X band are compared with related documents and FEM follows the main beam very well but disagrees more with the measurements away from the main beam. The theoretical analysis shows that the main beam of far-field radiation pattern shifts from axis direction about 27°～42° and the gain is about 11～14dB with 30° the bevel cut angle in C-band, it can be applied to high power microwave.