Monodisperse Au-Ag alloy nanoparticles were synthesized by a one-step rapid injection method in a capillary microreaction system with a short residence time (less than 3 min). Au-Ag alloy NPs possessed homogenous composition that revealed a linear relationship with surface plasmon resonance absorption peak. The optimal size distribution of Au-Ag alloy NPs with a standard deviation of 6% could be obtained from decreasing the molar ratio of Au3+ :Ag+ from 1:10 to 1:20 or enhancing the synthesis temperature from 140 to 160 degrees C. Transmission electron micrographs showed as-obtained Au-Ag alloy nanoparticles were sphere-like with the smallest average size of only 2.7 nm. Crown Copyright (c) 2012 Published by Elsevier B.V. All rights reserved.