The road friction is determined from the deviation of the estimated yaw rate under a standard condition from the actually measured yaw rate. The system for determining the road friction includes a parameter identification unit which determines a parameter of a variation term defined in the transfer function of the model for the vehicle response. The variation term may consist of a zero-th order or a first-order transfer function which would not cause any large computation load, and may not depend on the vehicle speed. The determined road friction may be used for controlling the rear wheel steering angle of a four wheel steering vehicle.