In this paper the mathematical model of a tilt-rotor aircraft is developed and the control system for vertical flight is studied. The modeling technique used here is based on that of a single-main-rotor hellicopter. Mathematical models are developed for each subsystem-fuselage, rotor, stabilizers, and main wing, and the mathematical model is obtained by assembling the subsystem of the vehicle models. The trim condition is found by the gradient method and the linear equation of motion is derived by assuming small perturbations. The control of pitch attitude, vertical velocity, and forward velocity is then studied for the helicopter-mode flight. Furthermore, the performance of the control system is investigated by conducting computer simulations.