In this study, the simultaneous vehicle-handling and lateral stability improvement of a Four-wheeled vehicle through an active robust control system is proposed. The control system is designed based on a simplified dynamic model and on the basis of sliding mode control method. The active steering control system is used to eliminate the the error between the actual and desired responses. In this control system, two state variables, namely the yaw rate and the lateral velocity of the vehicle are controlled as the target responses using active steering. Also, a fourteen degrees-of-freedom nonlinear dynamic model of a Four-wheeled vehicle is initially developed, and then it validated using CarSim software for standard maneuver. Then, the vehicle dynamic performance is evaluated for both uncontrolled and controlled states. The simulation results show the active steering control system not only improves the vehicle handling but also increases the lateral stability of the vehicle, especially under a critical maneuver in which considerable instability occurs.