Nickel nanoparticles have been prepared by the reduction of nickel chloride induced by γ-ray irradiation with a dose of 10~30 kGy and a dose rate of 3.9 kGy/hr at room temperature under atmospheric pressure. X-Ray diffraction (XRD) analysis revealed that the prepared particles of nickel had cubic phase. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and particle size analysis (PSA) indicated that these nickel nanoparticles had diameters of 10~15 ㎚ and a narrow size distribution. Also, we have shown that a number of parameters, such as the concentrations of nickel ions and isopropyl alcohol, the irradiation dose, and the types of surfactants, can be used to control the size of the nickel nanoparticles.