The microwave-induced plasma in liquid process (MWPLP) has been attractive as a unique preparation method of metal nanoparticles. In this study, we investigated the effect of H2O2 on gold nanoparticles prepared using the MWPLP. Au nanoparticles were obtained without aggregation when the Au and H2O2 concentrations in the reaction solution were 0.3 mM and 0.03 mM, respectively. When the H2O2 concentration was lower or higher than the optimum range, aggregated particles were obtained. The results show that the addition of H2O2 had a great influence on the reduction rate and the dispersibility, and that Au nanoparticles prepared under optimum conditions were not aggregated and exhibited a high dispersibility. (C) 2017 Elsevier B.V. All rights reserved.