To provide an improved decorative cap and a wheel nut or a bolt assembly, and to provide their assembling method.

An insert is coated and/or plated, and includes a hexagonal part and a part having a cylindrical outer surface. The cap includes a part having a hexagonal cross section and a part having a cylindrical cross section. The part of hexagonal cross section of the cap covers the hexagonal part of the insert, and is easily slid, and the part of cylindrical cross section has an inner diameter smaller than an outer diameter of the cylindrical insert part. The cap is made out of a plastically and elastically deformable sheet material. The cap is plastically and elastically deformed when it is pressed to the insert. The interference fit is formed therebetween by the circumferential plastic deformation of the cap.