In this paper, we investigate the problem of existence and asymptotic behavior of the solutions for the nonlinear boundary value problem ǫy + ky = f(t, y), t ∈ 〈a, b〉, k > 0, 0 < ǫ << 1 satisfying Neumann boundary conditions and where critical manifold is not normally hyperbolic. Our analysis relies on the method upper and lower solutions.