Spherical Cu nanoparticles were synthesized in sodium dodecyl sulfate (SDS)/isopentanol/cyclohexane/water microemulsions with sodium borohydride as a reducing agent. Nanoparticles formed in microemulsions were characterized by transmission electron microscopy (TEM), x-ray diffractometry (XRD), and small-angle X-ray scattering (SAXS). The size and polydisperity of particles were greatly affected by the mole ratio of water to surfactant (w) and the concentration of precursor salt. The ideal size and monodispersity of nanoparticles can be obtained at the smaller w value and lower concentration of Cu2+ solution.