Electrochemical preparation of Co-Sm nanoparticles was conducted in an aprotic room temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Co(TFSA)2 and Sm(TFSA)3. The cyclic voltammetry on a glassy carbon (GC) electrode indicated the electrochemically generated Sm(II) reacted with Co(II) at 25 °C. Potentiostatic cathodic reduction on a GC electrode in BMPTFSA containing 30 mM Co(TFSA)2 and 5 mM Sm(TFSA)3 at 25 °C gave the deposits, which were found to be composed of Co and Sm by energy dispersive X-ray spectroscopy (EDX). The deposits were found to be the aggregates of SmCo7 nanoparticles by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The formation of SmCo7 nanoparticles dispersed in the ionic liquid was also confirmed by TEM. SmCo7 nanoparticles were considered to form by the disproportionation reaction of Sm(II) in the presence of elementary Co, which was formed by the reduction of Co(II) by Sm(II).