Nano-sized gold particles were prepared in a solution containing polyethylenimine (PEI) utilizing a batch reactor. PEI acts as a reducing agent as well as a stabilizing agent of nanoparticles. The effects of initial concentration of PEI on the mean particle diameter, coefficient of variation (C.V.), growth rate, nucleation rate and the number of nuclei were studied. The particle diameter of gold decreased markedly with increasing initial concentration of PEI, from 3.3 mu m to a minimum value of about 5 nm. The observed decrease of particle diameter was considered to be caused by the growth-inhibiting effect of PEI, which affects the conclusive number of nuclei. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.