The fairish monodisperse Ag-Au alloy nanoparticles (NPs) with various composition and size were synthesized by a facile fabrication procedure. UV-visible (UV-vis) spectroscopy and transmission electron microscopy (TEM) confirmed the formation of homogeneous alloy NPs. The diameters of the resultant monodisperse spherical Ag-Au alloy NPs are 21-32 nm and the long-axis size of the resultant monodisperse nonspherical Ag-Au alloy NPs are about 28 nm. The possible formation mechanism is suggested by analyzing the dynamic absorption spectra, TEM and energy dispersive X-ray (EDX) spectroscopy of the alloy nanoparticles at the different time. Ag-Au alloy NPs synthesis method developed in this paper has several advantages: (1) the synthesis process is very simple. Only sodium citrate was used as both reducing agent and stabilizing agent and only one step heating process was needed. (2) The resultant Ag-Au colloidal dispersion is free from AgCI deposition and (3) the fairish monodisperse and large size (>20 nm) Ag-Au alloy NPs can be obtained. (C) 2011 Elsevier B.V. All rights reserved.