The grafted copolymer (SBS-g-MBP) with different grafting ratio, synthesized by our laboratory, was irradiated by ultraviolet (UV) radiation with different time. The structure of SBS-g-MBP before and after UV exposure was characterized by Fourier transform infrared spectrometer, and the results confirmed that the UV-cross-linked structure was formed. The gel fraction before and after UV exposure was discussed, and the results indicated that the gel fraction increased with both exposure time and grafting ratio, and it also indicated the gel fraction was affected mainly by the exposure time. Further-more, the results of scanning electron microscopy showed that the mode of fracture surfaces transmitted gradually from tough fracture to brittle fracture. The differential scanning calorimetry and thermal gravimetric analyzer indicated that the crosslinking has taken place in the polybutadiene block, and the thermal property was improved after UV exposure. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2627-2631, 2011