Nickel oxide (NiO) hollow fibers were obtained by thermal degradation of nickel alginate fibers (Ni-AF), which was prepared by wet spinning technique. The composition, morphology and microstructure of the as-obtained NiO fibers were investigated by thermal gravimetry (TG), X-ray diffraction (XRD), electron energy-loss spectroscopy (EELS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is found that temperature plays important roles on the final structures and shapes of the NiO nanostructures. NiO hollow fibers (∼ 10 μm) consisting of NiO nanoparticles with diameter about 50 and 80 nm respectively are obtained under 600 and 800 °C in air. The mechanism of Ni-AF thermal degradation and NiO growth were proposed.