Gold nanoparticles (AuNPs) with particle size below 10 nm exhibit high catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). However, AuNPs are unstable in aqueous solution or an organic solvent in the absence of a stabilizer and a dispersion agent. In this study, we prepared highly dispersed AuNPs using the ABC triblock copolymer poly(ethylene glycol)-block-linear polyethylenimine-block-poly(a-caprolactone) (PEG-PEI-PCL) as the stabilizer for the first time. The preparation route is very mild, convenient, and reproducible. The mean diameter of AuNPs is approximately 8 nm with a normal distribution. The as-prepared PEG-PEI-PCL-stabilized AuNPs have high catalytic activity for the reduction of 4-NP by sodium borohydride (NaBH4) in aqueous solution. Compared with AB diblock copolymer PEG-PEI, ABC triblock copolymer PEG-PEI-PCL as the AuNPs stabilizer has significant advantages in stability and catalytic activity. (C) 2015 Elsevier Inc. All rights reserved.