Nickel nanoparticles were prepared by reduction of nickel-hydrazine complex with sodium borohydride (NaBH4) in water-in-oil (w/o) microemulsion of PVP/ethanol/toluene/water at 50C. It was investigated through SEM and TEM photographs that the nickel nanoparticles with a diameter 15~20nm were nearly monodispersed. By the analysis of XRD, the resulting particles were characterized to be pure crystalline nickel with a face-centered cubic structure. The results indicate that the size, shape and distribution of particles depend on the mole ratio of [water]/[PVP] and [NaBH4]/[Ni]. Also, it was found that particle size could be controlled by the kinds of surfactants.