Two highly cross-linked superfine styrene-butadiene rubber particles, one with 1 wt% of carboxyl groups and theother without such groups having particle sizes of 130-150 nm and 80-100 nm respectively, were used to prepare nylon6/rubber composites via in situ polymerization. It was found that carboxylic styrene-butadiene dispersed uniformly in nylonmatrix and there was strong interfacial interaction because of the graft polymer formed by the reaction of nylon with carboxylgroup of the rubber, resulting in considerably improved impact strength with almost unchanged tensile strength. However,the addition of styrene-butadiene without carboxyl groups showed intensive agglomeration of the rubber particles and weakinterfacial interactions, and the toughness of the materials was improved slightly. The crystallization and rheological behavior of the composites were also discussed. Two highly cross-linked superfine styrene-butadiene rubber particles, one with 1 wt% of carboxyl groups and the other without such groups having particle sizes of 130-150 nm and 80-100 nm respectively, were used to prepare nylon 6 / rubber composites via in It was found that the carboxylic styrene-butadiene dispersed uniformly in nylon matrix and there was strong interfacial interaction because of the graft polymer formed by the reaction of nylon with carboxylgroup of the rubber, resulting in considerably improved impact strength with almost unchanged tensile strength. However, the addition of styrene-butadiene without carboxyl groups showed carboxyl agglomeration of the rubber particles and weak interfacial interactions, and the toughness of the materials was improved slightly. The crystallization and rheological behavior of the composites were also discussed.