The graft copolymerization of styrene onto low density polyethylene ( LDPE) with dicumyl peroxide as an initiator was studied. The existence of the graft copolymer was verified by infrared spectra. The effects of the initiator concentration, the feed composition, the reaction time, and the reaction temperature were discussed. Grafting efficiency reached a maximum value with the increase of reaction time. The effect of increasing the concentration of dicumyl peroxide was to decrease the grafting efficiency up to a minimum value. Grafting efficiency was not affected much by the variation of the reaction temperature. Grafting efficiency decreased largely with the increase of the feed composition of styrene. The compatibilizing effect of the copolymer prepared in this study was demonstrated by the morphology and mechanical properties of high impact polystyrene/LDPE blends.