In this work the particle size control method of copper nanoparticles synthesized in supercritical water was investigated. The experiments were respectively carried out in a flow-type reaction apparatus and in a batch-type quartz tube apparatus. Results show that particle size of the products was very sensitive to the concentration of copper sulfate and alkalinity of the reactant. When the concentration of copper sulfate increased from 0.05 mol/L to 0.5 mol/L, the average particle size increased from 14 nm to 50 rim. When the molar ratio of NaOH to copper ion increased from 0:1 to 2:1, the average particle size decreased from 85 nm to 14 nm. The homogeneity and dispersity of the products were improved by increasing the concentration of complexing agent ethylenediamine tetraacetic acid (EDTA). The adsorption mechanism of EDTA on the surfaces of nanoparticles was researched by a Fourier transform infrared spectroscopy. It was speculated that EDTA adsorbed on the surfaces of nanoparticles by the dehydration reaction of carboxyl groups in the molecule of EDTA and hydroxyl groups on surfaces of copper nanoparticles (-Cu-OH). These results are expected to be of great interest as basic data for the preparation of size-controlled copper nanoparticles by supercritical hydrothermal synthesis method. (c) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.