PURPOSE: To provide a mounting structure of a door trim, which can eliminate the formation of a gap between the front end part of the upper edge part of the door trim and a corner piece cover to improve the appearance.

CONSTITUTION: Engagingly locking projecting pieces 20 are installed to be projected toward the room side through a thin hinge part 23., along the longitudinal direction over the full length of the lower edge of a corner piece cover 7, while rib pieces 23 are installed to be projected on the underside of the front end part of the upper edge part 9a of the door trim 9 vertically at specified intervals. Engagingly locking projecting pieces 20 are inserted and engagingly locked between the front end part of the upper edge part 9a of the door trim 9 and the rib pieces 23 to fix the front end part of the upper edge part 9a of the door trim 9.