X-ray absorption fine structure (XAFS) and high angle annular dark field scanning transmission electron microscopy (HAADF STEM) were both applied to a characterization of the size regulated Ni nanocluster catalyst on alumina. XAFS analysis showed that NiAl2O4 was not formed in our colloid derived catalyst. The wide variety of coordination number (CN) for Ni-Ni indicated the achievement of the size regulated Ni nanocluster catalyst preparation. HAADF STEM analysis showed narrow size distribution of Ni nanocluster catalyst even after H-2 reduction at 673 K. Distorted particles were observed and the ratio of distorted particles increased with decrease in the cluster size. Water gas shift reaction was carried out to investigate the catalysis. Volcano-shaped size dependence was observed for the activity of supported Ni nanocluster catalyst. (C) 2014 Elsevier B.V. All rights reserved.