The reaction force and the bending moment of diaphragm couplings due to parallel-offset and angular misalignment were evaluated and the simulation method for the reaction variation occurring during the rotation was proposed using finite element analysis. Four kinds of the diaphragm, such as disk, hollow disk, disk with side cut off and hollow disk with side cut off, are modeled and used to calculate the reaction force and torsional stiffness. Since the variation of reaction force is sinusoidal waveform, the mean reaction force and reaction force amplitude could be determined. For estimating the transmission performance, the ratios of torsional stiffness to the mean reaction force and to the reaction force amplitude were proposed and used to discuss the effect of the diaphragm dimensions and the thickness. As a result, the performance of the diaphragm disk with a center hole is improved by cutting the disk side such as a square and thinning the diaphragm develops the performance.