In this paper, we analytically considered the effects of the in-plane stress of the disk on the changes of modal shape. The purpose of this study is to confirm whether the modal shape changes according to the in-plane stress. The results make it clear that the in-plane stress changes the modal shape and its curvature. The curvature of the modal shape is proportional to the reciprocal number of the bending stiffness. Thus, it is confirmed that the in-plane stress serves to change the bending stiffness distributions on the disk. Moreover, the natural frequencies of the disk are evaluated by two energy methods in which the variations of modal shapes are considered or not. The difference in the natural frequencies from two energy methods shows the effect of the changes in the modal shape of the disk due to the small difference in the results of the energy methods (maximum difference of strain energy: 7.1%). Finally, the two energy methods are compared to investigate the effects on the natural frequencies with hammering test. The experimental results show the traditional energy method is allowed to evaluate the natural frequencies practically.