Cobalt nano-particles have been prepared by reducing divalent cobalt species in an ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide at room-temperature. Potentiostatic cathodic reduction of divalent cobalt species was conducted on a platinum mesh electrode at −2.0 V vs. Ag/Ag(I) in the ionic liquid, resulting in the change of color of the ionic liquid from purple to black. The transmission electron microscope observation of the ionic liquid after the potentiostatic cathodic reduction showed formation of cobalt nano-particles with the diameters from 2 to 10 nm.