The decomposition of methanol-d4 on vanadium (V) nanoclusters grown by vapor deposition of V on an ordered thin film of Al2O3/NiAl(100) was studied under ultrahigh-vacuum conditions and with various surface probe techniques and calculations based on density functional theory. The V clusters had mean diameter 1.55-2.10 nm and height 0.46-0.66 nm evolving with their coverage; they grew in a bcc phase and primarily in orientation (001); the lattice contracted 4% (relative to the bulk) to match structurally better the alumina surface. The methanol-d4 adsorbed on the V clusters decomposed largely through the formation of methoxy-d3 (denoted as CD3O*) at 175-225 K and subsequent cleavage of the C-O bond, yielding methyl-d3 (CD3*), at temperature ≥350 K; CD3* either combined with surface deuterium (D*) and desorbed as methane-d4 (CD4(g)) or dehydrogenated to supply more D* to assist the formation of molecular deuterium (D2(g)). As a measure of the reactivity, the quantities of CD4(g) and D2(g) produced per surface V site exhibited an evident dependence on the cluster size. Both these productions were inhibited on small clusters but increased with the cluster size; the production of D2(g) per surface site was saturated about that from V thin films, whereas that of CD4(g) attained a maximum at a cluster diameter near 2.0 nm but decreased, with further increasing size, to a value near that from V thin films. We argue that the inactivity of small V clusters arose from an increased energy barrier for scission of the C-O bond of CD3O*, which is a critical step of methanol-d4 decomposition, on two-dimensional structures of small clusters; the separate trends of production of CD4(g) and D2(g) on larger clusters are attributable to the competition of CD3* and D* for D*.