In this study, a new supported cobalt nanocatalyst has been described. The Fe3O4 magnetic nanoparticles (Fe3O4 MNPs) modified by SiO2/aminopropyl trimethoxy silane/cyanuric chloride (Fe3O4@SiO2-APTMS/CC) utilized for anchoring metformin-cobalt complex (Fe3O4 Ms@SiO2-APTMS/CC/Met@Co(II)). The structure of novel complex well defined by elemental analysis, ICP, AAS, BET, FT-IR, EDX, SEM, TEM, DLS, XRD, TG-DTG, VSM and XPS. The catalytic efficiency of the synthesized cobalt nanocatalyst was studied in the oxidation of ethylbenzene (EB), cyclohexene (CYHE) and various oximes using molecular oxygen as ecofriendly oxidant and high catalytic activity and selectivity toward oxidation is observed. Selective aerobic oxidation of EB and CYHE and various oximes catalyzed by the cobalt nanocatalyst without any reducing agent by using N-hydroxyphthalimide (NHPI), gave acetophenone (AcPO), 2-cyclohexene-1 -one and corresponding carbonyl compounds respectively, as major products. To achieve high level of efficiency of heterogeneous nanocatalyst, various parameters such as the ratio and amount of nanocatalyst/NHPI, reaction time, temperature and solvents were evaluated. The easily preparation from inexpensive and commercially available reagent, thermal stability, suitable performance in reusability, high efficiency and selectivity in oxidation reactions, short reaction time, easy recovery and separation from reaction mixture, are advantages of this novel catalyst. (C) 2017 Elsevier Inc. All rights reserved.