Abstract: Electric power steering system comprises an electric motor (9) for producing a steering assist force reducing a steering effort of a vehicle operator and a control unit (21) determining a control value for the electric motor, the control unit being configured to determine the control value by selecting a map representing a relationship between a vehicle state variable and a control value for each of a plurality of control modes. When switching control modes, the control unit gradually changes the control value from that obtained from a map corresponding to a preceding control mode to that of a following control mode. A different time period of gradual transition from the preceding control mode to the following control mode is selected depending on those control modes.