The carbon supported AuNi electrocatalyst (AuNi/C) for electrochemical oxidation of borohydride is prepared in water-in-oil microemulsion of water/AOT/n-heptane. The effects of the molar ratios of water to AOT (R omega) on the particle size and catalytic performance are systemically investigated. It has been found that when the R omega is less than 10, the particle size increases with the increase of R omega value, and the particle size is decreased after R omega > 10 due to two-phase separation of the microemulsion. The morphology and structure of as-prepared AuNi/C electrocatalysts are examined by transmission electron microscopy (TEM) and X-ray diffractometer (XRD). It is confirmed that all AuNi nanoparticles are spherical with average diameters in the range of 3-9 nm and uniformly distributed on the surfaces of carbon. The electrooxidation behaviors of BH4- on the AuNi/C anodic electrocatalyst have been investigated. The results show that the catalytic activity of BH4- oxidation on AuNi/C electrocatalyst is apparently improved and the smaller particle size can get a higher catalytic activity. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.