We use the method developed earlier by Abrams and Henderson to deal with coupl-ing losses of modes in rectangular waveguide laser resonator. Coupling loss of EH_(11) modeon wavefront matching mirror and flat mirror have been calculated quantitatively. Thenthis method has been generalized to analyze self-coupling of higher order modes and cross-coupling of several modes. The diffraction loss due to definite aperture of mirrors hasbeen discussed. On the basis of these calculations the transverse mode discriminationof rectangular waveguide laser resonaror has been analvsed systematically and insights We use the method developed earlier by Abrams and Henderson to deal with coupl-ing losses of modes in rectangular waveguide laser resonator. Coupling loss of EH_ (11) modeon wavefront matching mirror and flat mirror have been calculated quantitatively. Thenthis method has been generalized to analyze the self-coupling of higher order modes and cross-coupling of several modes. The diffraction loss due to definite aperture of mirrors has been discussed. On the basis of these calculations the transverse mode discrimination of rectangular waveguide laser resonaror has been analvsed systematically and insights