Nonlinearities in mechanical systems demand extensive efforts both in modeling their dynamics and identification of their parameters. In this paper an identification approach is proposed for nonlinear multi-degree-of-freedom systems when there is a single nonlinear element in the system and its location is known. The proposed method is an extension of the frequency response function (FRF) sensitivity method used in the linear systems. The method uses measured nonlinear FRFs and employs a reduced order model for sensitivity calculations. Numerical and experimental case studies are used to verify the accuracy of the proposed method.