Superior catalytic activity of Au@PEG-b-PS was observed in the reduction reaction of 4-nitrophenol to 4-aminophenol. The catalytic activity increased with the decrease in the chain length of the PS block. In addition, the high stability imparted by the PS layer endowed Au@PEG-b-PS with good reusability in catalysis without the loss of catalytic activity, and prevented from electrolyte-induced aggregation, making the system very attractive as nanoreactor.