Size-dependence is an important factor in gold nanocatalysis. In this study, we explored the catalytic performance of atomically precise Aun(PET)m nanocluster catalysts (where, PET = phenylethanethiolate) of different gold atoms and sizes, including Au25(PET)18 (∼1.2 nm), Au38(PET)24 (∼1.5 nm), and Au144(PET)60 (∼1.9 nm) nanoclusters. These Aun(PET)m gold clusters, immobilized on activated carbon (AC) and used as heterogeneous catalysts, were characterized by transmission electron microscope (TEM), BET as well as X-ray photoelectron spectroscopy (XPS). They showed good catalytic activity in the aerobic oxidation of d-glucose into gluconic acid (or gluconates) with ∼98% selectivity. We observed a distinct size dependence of the gold nanocluster in the oxidation reactions, which follows as Au144(PET)60/AC > Au38(PET)24/AC > Au25(PET)18/AC. It was primarily determined by the surface area of the nanoscopic Au nanocluster. Further, the turnover frequency (TOF) for the Au144(PET)60/AC catalyst was found to be 2.3 s-1, which is comparable with that for Au/EC300 and much higher than those for the commercial Pd/AC and Pd-Bi/AC catalysts under the identical reaction conditions. On the whole, the core size of the gold nanoclusters played an important role in the catalytic process.