The article summarizes our recent advances on the size specific catalysis by polymer-stabilized gold clusters. We found that the smallest polymer-stabilized gold clusters (diameter = 1.3 nm) exhibit excellent activity for oxidation of alcohol, H₂O₂ generation from ammonium formate and molecular oxygen, and hydroxylation of dibenzylketones, by utilizing a molecular oxygen as an oxidant. The activity increases dramatically with decreasing the size down to 1 nm, which is closely correlate electronic structure of the smallest gold clusters. Various spectroscopic characterizations revealed that the smallest, active gold clusters are negatively charged through electron transfer from surrounding polymers, and as a result adsorbed molecular oxygen can be activated for the oxidation reaction through transfer of the excess electrons from the gold clusters, which are located in the high lying LUMO level of gold in the case of the smallest clusters.