The nonlinear dynamic behaviour of a microresonator driven by electric force was investigated. The first three modes were used to get a model with singledegreeoffreedom in terms of the nonlinear Galerkin method. The multiple scales method was employed to obtain the dynamic responses of the reduced order model. The amplitudefrequency curve of stationary responses was depicted out and used to compare with the results obtained by the traditional Galerkin method where only the first mode is considered. Taking the dynamic responses obtained numerically from the model with threedegreesoffreedom as a reference, it can be concluded that the nonlinear Galerkin method has higher accuracy in comparison with the traditional Galerkin method.