Grafting of maleic anhydride (MAH) onto isotactic poly butene-1 (iPB-1) was carried out by thermal decomposition of dicumyl peroxide (DCP) using electron-donating monomer styrene (St), and were carried out in the molten state in a twin-screw extruder according to an experimental design in which the content of MAH and St were varied. The calibration curve was constructed from FTIR measurements and titration which can obtain the absolute amounts of grafted MAH according to FTIR data. The proposed mechanism was that when St is added to the iPB-1/MAH/peroxide grafting system, St reacted first with MAH to form a charge-transfer complex (CTC). Then CTC react (or copolymerize) with macroradicals. The grafting of MAH onto iPB-1 (iPB-1-MAH) accelerated crystalline transformation rate of form II to I. The contact angle decreased with the increase of grafting degree, which indicated that surface polarity increased. POLYM. ENG. SCI., 51:2483-2489, 2011. (C) 2011 Society of Plastics Engineers