Abstract: Core-shell PB-g-PS impact modifiers with different mass ratios of polybutadiene (PB) to polystyrene (PS) were prepared with emulsion polymerization by grafting styrene onto polybutadiene latex. The mass ratio of polybutadiene to polystyrene was in a range from 30/70 to 90/10. Then these core-shell impact modifiers were blended with polystyrene to prepare the PB-g-PS/PS blends with a constant PB content of 20wt%. The grafting degrees of PB-g-PS copolymers were measured and the effects of different grafting degrees on the dispersion state and morphologies of rubber particles are discussed. It was generally accepted that polystyrene could only be toughened effectively by 1~3μm rubber particles. However, the experimental results of this paper show that PS can be actually toughened by sub-micron core-shell rubber particles. The optimal degree of grafting is 28.3% and the ratio of PB/PS in PB-g-PS copolymer is 70/30. The TEM photograph of PB-g-PS/PS (70/30) shows that the rubber particles disperse homogeneously in the PS matrix, and the izod notched impact strength attains 124.9J/m.