The preparation of nickel and iron nanoparticles was achieved in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifloromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)2 or Fe(TFSA)2. A potentiostatic cathodic reduction of Ni(II) in Ni(TFSA)2/BMPTFSA at - 2.6 V made the color of the electrolyte change from yellow to black and resulted in formation of Ni nanoparticles, which were confirmed by transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). Addition of acetonitrile (ACN) into Ni(TFSA)2/BMPTFSA resulted in a reduction of the mean diameter of Ni nanoparticles from about 3 nm to 1 nm. This may be due to the changes in the coordination environment of the Ni(II) species in the ionic liquid, from [Ni(TFSA)3]- to [Ni(ACN)6]2+. The mean diameter of Fe nanoparticles, which were prepared from Fe(TFSA)2/BMPTFSA, was close to that of Ni nanoparticles.