A parameter called the change rate of the mode axial displacement difference is proposed based on the feature of vibration mode, and is used to judg crack of roller. Displacement mode of roller is calculated by establishing the three-dimensional finite element models of roller with no crack, external crack and inner crack, respectively. The influence of the position and depth of crack on the change rate of the axial displacement difference in the first mode of roller with external and inner crack is also analyzed by extracting the axial displacement in the first mode of a generatrix on the surface of the roller. The results demonstrate that the change rate of the axial displacement difference in the first mode exist a peak at the crack position. And the peak goes higher when the crack is deeper. Therefore, it is feasible to identify the roller's crack damage by the change rate of the axial displacement difference in the first mode.