Starch was grafted by poly(methyl methacrylate) through the emulsion polymerization method. Modified starch/(acrylonitrile-butadiene rubber) (NBR) compounds were prepared by a latex blend method. The morphology, thermal properties and mechanical properties of the modified starch/carbon black/NBR composites were investigated with the change of starch concentration. The mechanical properties of the composites were improved by the addition of modified starch. But, when the concentration of modified starch was higher than 40 phr, the mechanical properties were deteriorated due to the poor dispersion of modified starch. At the same ratio of starch to carbon black, the composite showed a synergistic reinforcing effect by the good dispersion and high cross-linking density. In addition, the tensile strength, storage modulus, hardness, swelling and other properties were the best.