In this paper, for the purpose of evaluating the degradation level of the performance in application with the parametric model in which the estimated parameters have the disturbance, we consider the relation between the robustness of parameter estimation and the model order. After the application performance cost function is derived, the gradient at the estimated parameter is focused to evaluate the disturbance level of the performance in the application. As a result, in this paper, it is shown that the robustness of parameter estimation is given as 2nd order function of the model order.