Gold nanoparticles with uniform mean sizes (approximate to 3nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the beta C-H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step. ; National Natural Science Foundation of China[20625310, 20773099, 20873110, 20923004]; National Basic Research Program of China[2010CB732303]; Research Fund for the Doctoral Program of Higher Education[20090121110007]; Key Scientific Project of Fujian Province[2009HZ0002-1]