A power seat slide apparatus for a vehicle includes a first rail adapted to be fixed to a vehicle floor, a second rail adapted to be fixed to a vehicle seat and slidably supported relative to the first rail, a screw shaft screwed to a nut member which is fixed to the first rail, the screw shaft including an end portion and a stepped portion, a washer engaged with the stepped portion of the screw shaft, a gear member to which the end portion of the screw shaft is inserted, a fastening member provided at the end portion of the screw shaft, and a gear box fixed to an end of the second rail. The gear box includes a support bracket, a gear housing, and a plate. The stepped portion of the screw shaft is arranged in such a manner that the washer is connected thereto in the gear housing.