In geodesy many models are nonlinear models. The classical method dealing with these nonlinear models is linear approximation using the approximate value of parameter. Due to the difference of nonlinearity between different nonliear models, some nonlinear models can be linear approximation and the others can be not. How to measure nonlinearity of nonlinear models? This paper introduces a method to measure nonlinearity of nonlinear mdoel and presents a numerical standard for linear approximation-acceptable curvature.