A novel magnetic catalyst was synthesized by the covalent grafting of poly(1-vinylimidazole) onto the surface of Fe3O4 (MNP@PVIm). The resulted catalyst was used in the synthesis of 4H-benzo[b]pyrans by three-component reaction under aqueous condition and the products were obtained in excellent yields. The catalyst showed appropriate thermal stability and excellent recyclability. Because of the multi-layered form of coated polymer, the catalyst has high loading level of imidazole groups and is used in the low weight percent, comparing to substrates. After completion of reaction, catalyst is recovered in a green way using an external magnet. (C) 2014 Elsevier B.V. All rights reserved.