A motor-driven power steering apparatus and method for a vehicle are able to avoid frequent on-off operations of an electromagnetic clutch 14 as much as possible, which connects or disconnects a motor 13 with or from a steering system, thus improving the durability of the clutch 14 and suppressing the generation of undesirable noise and shocks upon connection and disconnection of the clutch 14. The motor 13 is deenergized by a motor controller 191 when the vehicle speed V is equal to or greater than a predetermined value Vo. The clutch 14 is turned off to disconnect the motor from the steering system when an abnormality in the steering system is determined during high speed travel of the vehicle. Such an abnormality is determined if the motor current flowing through the deenergized motor is not equal to zero, or if the steering torque is equal to or greater than a predetermined value and a voltage across the motor when deenergized is equal to zero, or if the rotational speed of the steering wheel is equal to or greater than a predetermined value and the motor voltage is equal to zero.