The invention provides a method for preparing nickel nano-particles and the nickel nano-particles produced by reduction after forming a nickel-hydrazine complex in a reverse micro emulsion by the method. The nickel nano-particles have excellent dispersion stability and smooth surface. The invention also provides a method for preparing nickel nano-particles with narrow particle size distribution less than 100 nm especially between 10 nm and 50 nm and the nickel nano-particles prepared by the method. According to one side of the invention, the method comprises the steps of: (a) forming a solution containing nickel precursor, a surfactant and a hydrophobic solvent; (b) forming the nickel-hydrazine complex by adding hydrazine containing compound into the mixture; (c) adding a reducing agent into the mixture containing the nickel-hydrazine complex to prepare the nickel nano-particles.