The robust control for nonlinear feedback system with unknown perturbations is discussed based on the robust right coprime factorization and passivity property. Firstly, based on passivity property, the right factorization of the perturbed plant is realized. Secondly, a robust control scheme is proposed considering the tracking property for the nonlinear system with nominal plant and perturbed plant. Finally, we show the connection between the passivity and robust right coprime factorization for the obtained nonlinear system. A numerical example is given to show the effectiveness of the proposed design scheme.