The problem of identifying nonlinear systems is proposed in the presence of hard nonlinearity. Presently, the nonlinear system is structured by Wiener-Hammerstein systems consist of a series connection including a nonlinear element sandwiched with two linear subsystems. Presently, the two linear subsystems are allowed to be nonparametric and structure entirely unknown, but are supposed asymptotically stable. Furthermore, the system nonlinearity is of hard type and no knowledge is priori required. Interestingly, the system nonlinearity is separately identified first. In turn, the linear subsystems are identified in the second stage using a frequency approach.