A steer angle control system for a controlled vehicle comprises sensors for sensing the steering wheel angle and the vehicle speed of the controlled vehicle, a processor such as a microcomputer, and an actuator for steering the front and/or rear wheels of the controlled vehicle in response to a control signal produced by the processor. The processor includes a reference behavior determining section for determining the value of a reference output motion variable corresponding to the sensed steering wheel angle and vehicle speed by solving a reference mathematical model representing desired vehicle motion characteristics, a vehicle behavior monitoring section for determining a value of the plant output motion variable which is a linear combination of a yawing motion variable, such as a yaw rate of the controlled vehicle, and a side translational motion variable, such as a lateral acceleration of the controlled vehicle, and a controller section for producing the control signal representing a target steer angle so as to reduce an error between the reference variable and the plant variable.