Abstract: In order to obtain nickel oxide (NiO) nanowires with controllable grain size, a method to fabricate NiO nanowires via high temperature oxidation is presented, and the specific oxidation mechanism is investigated with XRD, SEM, TGA and HRTEM approaches. Microstructure characterization indicate that the average grain sizes of NiO nanowires are about 12nm (400℃, 5 h), 16 nm (600℃, 5 h), 21 nm (800℃, 5 h), respectively, which illustrate that the average grain sizes of the NiO nanowires are getting bigger with the increase of oxidation temperature. Through the SEM analysis, NiO nanowires have similar length (150 μm) and diameter (150 nm) with the Ni nanowires. However, NiO nanowires have more smooth surface and straightness. TGA curves and HRTEM images indicate that the high temperature oxidation process from Ni nanowires to NiO nanowires must undergo a process of an amorphous oxidation.