Misalignment of a rectangle mirror resonator is discussed. Determination of axis and approximation of diffraction loss of the misalignment resonator are given for three lasers :the He2Ne laser、the CO2 laser and the CO laser. It is argued by the computation that the diffraction loss of the rectangle mirror resonator is decided by the misalignment degree of the rectangle short-side. The choice of the spheral mirror’s radius of curvature also affects the diffraction loss of the rectangle resonator.