In the present paper nonlinear normal modes of a rotor system is studied. The methodology to obtain the nonlinear normal modes is based on center manifold reduction technique. It also provides a way of nonlinear coordinate transform from the physical cordinates to the modal coordinates and an idea of individual nonlinear modal dynamics. In order to apply the present method to a rotor dynamics a single mass rotor system on nonlinear elastic supports is employed and the nonlinear normal modes of the system are obtained.