(In1-xNix)(2)O-3 (x = 0.03 and 0.06) powders were prepared by a solid-state reaction and a vacuum annealing process. The air-sintered samples initially showed paramagnetism, and then exhibited obvious room temperature ferromagnetism after vacuum annealing. The ferromagnetic signal almost disappeared after air annealing and reappeared after vacuum annealing. Hence, the ferromagnetism can be switched "on" and "off' by alternate air and vacuum annealing. X-ray diffraction, high-resolution transmission electron microscopy, and field-cooled/zero-field-cooled measurements were performed. These analyses confirmed that there was no detectable trace of Ni or Ni oxide secondary phase in vacuum-annealed (In1-xNix)(2)O-3 samples, and that the Ni ions were incorporated into the indium oxide lattice (substituting the In3+ ions). No ferromagnetic signal emerged from pure In2O3 powders regardless of undergoing vacuum or air annealing. These results revealed that oxygen vacancy and local spin are two factors that indispensably affect the ferromagnetic properties of the Ni-doped In2O3 system. (C) 2011 Elsevier B.V. All rights reserved.