Abstract Cobalt nanoparticles with a face-centered cubic (fcc) structure were synthesized by the reduction of Co(NO 3 ) 2 in supercritical methanol. The reduction of Co(NO 3 ) 2 to fcc Co was completed within 15 min at 400 °C and 300 bar. The reduction mechanism was investigated by varying the reaction temperature (200–400 °C) and the reaction time (0.5–15 min). The results suggested that cobalt methoxynitrate formed at an initial stage was converted to CoO, and then reduced to Co. In addition, an introduction of oleic acid as a stabilizer achieved the formation of Co nanoparticles (∼10 nm).