A phosphorus-on-skeleton compound was synthesized by reacting phenyl dichlorophosphate (PDCP) with 2-hydroxyethyl methacrylate (HEMA). This monomer was then copolymerized with other acrylic monomers to form a hydroxy-containing copolymer, which was then used as the polyol in the synthesis of a polyurethane. Phosphorus-on-pendent copolymers and phosphorus-free copolymers and their corresponding polyurethanes were also prepared for comparison with the phosphorus-on-skeleton material in terms of their flame-retardant properties. The flame retardancy and degradation mechanism of these copolymers and polyurethanes were analyzed with thermogravimetric analysis (TGA) and infrared spectroscopy. Although those phosphorus-on-skeleton copolymer polyols have less flame-retarding ability than that of the phosphorus-on-pendent copolymer polyol because of less phosphorus content, it was evident that the phosphorus-on-skeleton polyurethanes were more effective flame retardants than the phosphorus-on-pendent polyurethanes. This was attributed to the fact that the crosslinking arising from the phosphorus-on-skeleton copolymer polyols has a tremendous effect on the flame-retarding ability of the corresponding polyurethanes.