AbstractReactive compatibilization of an isocyanate (NCO) functional group as a reactive group was investigated. Five kinds of reactive polymers were prepared through solution graft polymerization of low‐density polyethylene (LDPE) copolymerized with acrylic acid (AA), maleic anhydride (MAH), glycidyl methacrylate (GMA), and masked and naked NCO functional monomers and were used as compatibilizers in poly(ethylene terephthalate) (PET)/LDPE and polyamide 6 (Nylon 6)/LDPE blends. The reactivity of the NCO functional group in the reactive blend was evaluated in terms of the blend morphology, adhesive test, mechanical properties, and rheological properties. It was found that GMA and NCO groups were the most effective compatibilizing agents in the PET/LDPE blends and MAH and NCO groups showed effective compatibilization of the Nylon 6/LDPE blends. We confirmed that the NCO functional group as a reactive group could be used as a universal reactive compatibilizer for PET/LDPE and Nylon 6/LDPE blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2622–2629, 2003