Abstract: Flexible air vehicles may encounter aeroservoelastic instability which is induced by the adverse coupling between structure, aerodynamics and controllers. Aeroservoelastic stability on roll loop of a missile with differential rudders and control system is analyzed. The generalized unsteady aerodynamics is calculated by the aerodynamics derivative method which is based on the quasi-steady aerodynamics theories. And the rigid-body roll and the first elastic torsion mode of missile body are concerned in the equations of dynamics. The open-loop transfer function of the roll loop is solved from the equation of dynamics, and the aeroservoelastic stability of the system is analyzed by using the Nyquist method. The results of a missile show that, the closed-loop aeroservoelastic system may be unstable when encounters ill-suited control design. So the elastic modes and frequencies of missile body should not be overlooked in missile design.