Gold particle nucleation and growth were analyzed systematically by UV-vis, ELS and TEM measurements. The gold seed particles ranging from 14.3 nm to 20.3 nm were prepared with monodisperse size distribution by citrate reduction reaction of various initial HAuCl4 concentrations. Gold nanoparticles of different sizes were made by the seed-mediated growth process. In the first stage of seed-mediated growth, most of gold precursors were used for the growth of seed particles, but, further supply of gold precursors promotes the nucleation of small particles and decreases the average particle diameters while the polydispersity of particles increases. This research can be used as basic data to prepare gold nanoparticles with specific properties for different applications. (C) 2009 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.