This paper is concerned with the robust stability and output tracking problems of nonlinear feedback control systems with nonlinear unstable plants with perturbations. Based on isomorphism approach, the right coprime factorization of the unstable plants can be realized, moreover, the robust stability of the whole nonlinear feedback control systems is guaranteed. Then the right coprime factorization is exactly a robust right coprime factorization of the nonlinear plant. Further, the designed tracking controller ensures the plant output tracking to the reference input. The effectiveness of the proposed method is confirmed by the simulation results.