In [2] M. Jaoua et al. studied the linear approximation of Robin problem on Ω an open bounded domain of R d , and they given some important results. In this paper, we study a nonlinear approximation of an elliptic problem with a nonlinear Robin boundary condition in a domain of R 2 . We prove the existence and uniqueness of solution by an iterative construction method with admissible condition on ∂Ω.