This work presents a simple and versatile grafting method to prepare stable polymer layers on poly(ethylene terephthalate) (PET). Poly(styrene-co-2-hydroxyethyl methacrylate) was synthesized by free radical polymerization. The copolymer was grafted onto poly(ethylene terephthalate) via the hydroxyl groups upon thermal annealing. The percentage of copolymer grafting was studied as a function of copolymer composition, PET surface treatment, annealing time, and annealing temperature. The grafted polymer layer on PET remained stable when immersed in an organic solvent whereas the non-grafted polymer layers were completely desorbed within two weeks. In addition, poly(4-vinylpyridine-co-2-hydroxyethyl methacrylate) was grafted onto PET to test the versatility of this grafting method. Control experiments were carried out to demonstrate that the grafting occurs through the available hydroxyl groups on the polymers and the surfaces. Based on these results, this grafting method offers a simple and versatile strategy to prepare stable polymeric layers on polymeric surfaces, and therefore, expands the tool box for functionalizing polymer surfaces.