Abstract: A novel method for parameter identification of structural nonlinearity is developed. This method takes advantage of the modal test and analysis, and could construct a more accurate model to represent the nonlinear behavior of the structure. The method consists of two dynamic tests: constant-amplitude displacement test and constant-amplitude velocity test. The two tests all use special sinusoidal force excitations. The constant-amplitude displacement test is used to identify nonlinear stiffness terms from measured frequency response functions of the structure subjected to sinusoidal force excitations which need to ensure the amplitudes of displacement is constant through adjusting the amplitudes of the force over the frequency range. The constant-amplitude velocity test is similar to the constant-amplitude displacement and used to identify nonlinear damping terms. A simulated 3DOF lumped-mass system with a weak nonlinear stiffness and damping behavior is used to verify the method.