A dynamical model of the airfoil with cubic and freeplay nonlinearity in both pitch and plunge was established. The nonlinear aerodynamic force and moment on the wing were evaluated by the third order piston theory. A bifurcation diagram, which presents the relevance between bifurcation parameter and amplitude of the periodic motion, was given numerically. When the dimensionless flight speed increases to the flutter critical speed, limited cycle can be observed in both pitch and plunge, and Hopf bifurcation appears, which means the stable point bifurcates to a periodic motion. And when further increasing the dimensionless flight speed a more complex dynamic phenomenon will turn up .