Natural rubber (NR) latex was grafted by emulsion polymerization with styrene monomer, using cumene hydroperoxide/ tetraethylene pentamene as redox initiator system. The polystyrene-grafted NR (PS-g-NR) was hydrogenated by diimide reduction in the latex form using hydrazine and hydrogen peroxide with boric acid as a promoter. At the optimum condition for graft copolymerization, a grafting efficiency of 81.5% was obtained. In addition, the highest hydrogenation level of 47.2% was achieved using a hydrazine:hydrogen peroxide molar ratio of 1:1.1. Hydrogenation of the PS-g-NR (H(PS-g-NR)) increased the thermal stability. Transmission electron microscopy analysis of the H(PS-g-NR) particles revealed a nonhydrogenated rubber core and hydrogenated outer rubber layer, in accordance with the layer model. The addition of H(PS-g-NR) at 10 wt % as modifier in an acrylonitrile-butadienestyrene (ABS) copolymer increased the tensile and impact strengths and the thermal resistance of the ABS blends, and to a greater extent than that provided by blending with NR or PS-g-NR.