Abstract: The blade of a large horizontal axis wind turbine is a nonlinear periodic multi-body system with fluid-rigid-flexible coupling. The blade motion stability involves the effects of gravity, the blades pre-cone and the rotating speed. The blade has elastic flap-lag-torsion and axial deflections along with rigid displacements of the flap, lag and pitch hinges. A new 5 node 18 DOF rigid-elastic beam element with three rigid displacements and fifteen nodal displacements is used to model the blade. The dynamics equations are derived using the Hamilton principle. The perturbation equations are then derived and solved using the Newmark integration method. The Floquet theory is used to analyze the blade stability by calculating the eigenvalues of the transition matrix. The results show that the effects of the blades pre-cone on the motion stability cannot be neglected without wind load and the isolated blade is stable when the turbine has a normal angular speed.