We have synthesized Mn-Pt nanoparticles using a chemical preparation based on a liquid-phase reaction caused by decomposition of organometallic precursors. The synthesized monodisperse Mn52.5Pt47.5 nanoparticles show an fcc crystalline structure and ferromagnetic properties even at the room temperature. Mn52.5Pt47.5, which is not ferromagnetic in a bulk form, shows a completely different physical property in nanoparticles.