The preparation of nickel particles from nickel ammine complex was studied by ultrasonic spray pyrolysis in the presence and the absence of hydrogen. In the presence of H-2 (about 9 vol.%), nickel particles were formed at 500 degreesC, which is much lower than that reported. Metallic nickel, together with NiO, was obtained as a major phase at 400 degreesC. In the absence of H-2, metallic Ni was also obtained, but higher temperature (e.g. 900 degreesC) was needed. It is suggested that the addition of NH3.H2O and NH4HCO3 to NiCl2. 6H(2)O precursor changes the reaction pathway of Ni formation. By careful control, spherical, solid and well-distributed Ni particles of about 0.5 mum were obtained at a residence time of 7-9 s in the aerosol reactor.