A structure of a fuel filler door for a vehicle includes a door hinge having one end portion coupled to a vehicle body, a support unit including a support plate coupled to an opposite end portion of the door hinge and a sidewall protruding from an edge of the support plate, and a cover plate coupled to an upper portion of the sidewall. A first locking step and a second locking step protrude from the sidewall positioned at opposite end portions of the support plate, and a locking member, which is rotatably locked to the first locking step, and a first hook member, which is fixedly locked to the second locking step, protrude from an inner surface of the cover plate.