Aeroelastic stabilities of the missile control fin with a pneumatic actuator are studied. The simple nonlinear model of a pneumatic actuator considering compressibility is derived to consider actuator dynamics, and the aeroelastic analyses of the missile control fin in supersonic range are performed. Supersonic 3-D unsteady aerodynamics of the missile control fin are computed by the panel method(DPM) in the frequency-domain, and are transformed to the Laplace domain aerodynamics for the time-domain aeroelastic analyses. The modal approach is used to save memories and computational times, and the fictitious mass method is used for the effective and accurate analyses considering an actuator. The effects of a pneumatic actuator with nonlinearity on the flutter characteristics of a missile control fin are studied and the limit cycle oscillation (LCO) of a control fin due to actuator nonlinearity is observed.