A control method of a rear wheel steering system prevents a sudden change in a rear wheel steering control amount when an error occurs in a wheel speed sensor. The control method detects an error in wheel speed sensors based on output values of the wheel speed sensors. A vehicle speed is estimated by using output values of remaining normal wheel speed sensors except for an output value of an abnormal wheel speed sensor where an error is detected, when an error in the wheel speed sensor is detected. Rear wheels are steered and controlled based on the estimated vehicle speed.