The NiOx thin films were deposited by reactive direct current (DC) magnetron sputtering from a nickel metallic target in Ar + O2 mixed gases with the relative O2 content 5%. The effects of heat treatment on the microstructure and optical properties of NiOx thin films were studied by X-ray diffraction (XRD), scanning electron microscope (SEM), atom force microscopy (AFM) and spectrometer respectively. In addition, the powders of the as-deposited NiOx thin films were investigated by thermal analysis. The results showed that the decomposition temperature of the as-deposited NiOx thin films was at about 262 ℃. After annealed at 400 ℃ for 30 min in air, the reflectivity decreased and transmittance increased obviously. XRD and differential scanning calorimetry (DSC) analysis show that no phase transformation occured during the heating process, the changes of optical properties were related to the decomposition of NiOx thin films. The activation energy of decomposition of NiOx thin films was 230.46 kJ/mol calculated by Kissinger formula, and the NiOx thin films have an excellent thermal stability. Its thermal stability and high optical contrast at 405 nm before and after annealing made it a good potential optical storage medium in write-once blue-ray disc.