We have synthesized single crystals of a 2-phenylethanethiol-protected Ni6 nanocluster, namely Ni6(C8H9S)12, which shows a hexagonal metallic core structure and reasonable stability. Interestingly this cluster is found to be an excellent candidate for the catalytic oxidation of toluene to benzaldehyde, with 100% conversion and 91% selectivity, showing application potential as an aromatic aldehyde in industry. Using DFT calculations, we rationalize the catalytic reaction mechanism for the conversion of toluene to benzaldehyde, and demonstrate that the presence of H2O2 initiates the Ni6 cluster via a highly exothermic step to form a Ni6O* intermediate which then results in active sites for the oxidation of toluene. What is interesting is that the likely steric geometry matching of the six-ring Ni6 core, a dozen phenylethanethiol ligands and the aromatic reactant toluene could attain a synergistic effect to facilitate the production of benzaldehyde free of catalyst support.