A steering control system for a self-propelled working vehicle having steerable front and rear wheels. The control system comprises sensors for detecting a directional or orientation error and a sideways error of the vehicle relative to a reference line or lines extending in a vehicle running direction, and a control unit for steering the vehicle in response to signals received from the sensors. The control unit is capable of steering the vehicle in a turn steering mode for turning the front and rear wheels to the same steering angle in opposite directions and a parallel steering mode for turning the front and rear wheels to the same steering angle in the same direction. When the directional error and the sideways error take place simultaneously, the control unit determines directions and angles for turning the front and rear wheels to correct the two errors simultaneously.