In this thesis a robust autopilot for a surface-to-air missile for different situation of flight is designed. The missile is tail control type. According to various altitudes and speeds of flight, different transfer functions for the system can be obtained as a designed invariant time controller can stabilize all models. On this purpose the transfer function of lateral acceleration to the input of finest deflection is separated to two transfer functions: first the transfer function of lateral acceleration to pitch angular velocity, second the transfer function of pitch angular velocity to finest deflection. It is shown that with this separation, having the transfer function of pitch angular velocity to controlled signal in hand, we can obtain the transfer function of lateral acceleration to command of proper acceleration. Since the system is time variant, the robustness of designed controller is proved theoretically and is confirmed with simulation.