Abstract: A mathematic model for calculating the ellipticity of the output beams in slightly noncoplanar resonators is given. By analyzing the error in the patent registed by R. H. Moore and taking the stress effect of the output mirror into account, the ellipticity of the output beams was analyzed. And it is found that the stress effect of the output mirror has the unsymmetrical influence on the clockwise and counterclockwise ellipticities of output beams. Moreover, the validity of the theoretical analysis is vertified by the experiment. Based on these results, the method proposed by R. H. Moore, S. W. Hammons et al.for aligning the light path can not reduce the magnetic effect by controlling the distortion angle of square ring resonators effectively.