Gold nanoparticles (AuNPs) covered with a series of well-defined poly(ethylene glycol)-b-polystyrene (PEG-b-PS) amphiphilic diblock copolymers containing a thiol group at the end of PS block were prepared to explore the influence of chain length of PS segment on the colloidal stability and catalytic activity of AuNPs. PEG-b-PS amphiphilic diblock copolymers with different PS chain lengths and narrow molecular distributions (M-w/M-n = 1.15) were synthesized by reversible additionfragmentation chain transfer (RAFT) polymerization employing a PEG-based macromolecular chain transfer agent (M-n <= 2000 g/mol), followed by transforming the thiocarbonate end functionality into a thiol group in the presence of 2-aminoethanol and tributylphosphane. PEG-b-PS-stabilized gold nanoparticles (Au@PEG-b-PS) were prepared by ligand exchange reaction between citrate-stabilized AuNPs and the thiol end group of PEG-b-PS diblock copolymer. The presence of the hydrophobic PS layer not only improved the stability of Au@PEG-b-PS against electrolyte-induced aggregation but also greatly promoted the resistance of Au@PEG-b-PS against competitive displacement of dithiothreitol. Au@PEG-b-PS showed excellent catalytic activity in the reduction reaction of 4-nitrophenol into 4-aminophenol, and the catalytic activity increased with the decrease in the chain length of 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.