A ring assembly for a pen has a bottom ring, a top ring, and a slender bridge connecting the bottom ring and the top ring. In assembly, a barrel of the pen extends through the bottom ring and the top ring, so that a sticker adhered around the barrel is securely attached to the barrel by clamping of the top ring and the bottom ring. Furthermore, a gap formed by two conjunct sides of the sticker is concealed by the bridge. By such arrangement, the ring assembly is able to be decorative and to avoid peeling of the sticker from the barrel after long-term use.