Abstract: An analysis method of the airfoil flutter system with nonlinear structure at transonic speeds is presented. The unsteady aerodynamic forces are calculated by Navier-Stokes equations. The time histories of structure responses of airfoil are solved by coupling the Navier-Stokes equations and flutter equations. The systems with gap stiffness, cubic type stiffness and cubic structure damping are investigated. The calculated results show that the influence of the structure nonlinearity on flutter speed is sensitive. Due to both the structure and aerodynamic nonlinearity in the flutter system, the oscillation characterstics become very complex.