A molded thermoplastic lug nut cap suitable for mounting wheel covers includes a hollow cylindrical body. Axially projecting from the body is a crown having a plan configuration congruent with a lug nut. The fastener is slipped over a lug nut which extends through an opening in the wheel cover and abuts against the wheel cover. A lug wrench is employed to twist the cap relative to the lug nut. The internal walls of the body include a plurality of chordal planar faces extending parallel to the axis of the cap with diagonally opposed chordal faces being positioned apart a distance greater than diagonally opposed faces of the lug nut but less than the distance between diagonally opposed corners of the lug nut. When the cap is twisted relative to the nut, corners of the nut engage and lock against the chordal faces in interference fit. An abutment rib project radially inwardly from each chordal face and provides a positive rotation stop when contacted by the face of the nut.