In this work, Ni(acac) 2 nanostructure was prepared through sublimation method that is a new method for the synthesis of nanomaterials. The temperature of sublimation is an effective parameter on morphology of products, so the role of this parameter on morphology and size of Ni(acac) 2 nanostructures was investigated. The uniform Ni(acac) 2 nanostructures were considered as an appropriate precursor for synthesis of nickel oxide nanoparticles (NiO NPs) by thermal decomposition method. Finally, NiO/ graphene nanocomposite (NiO/G) was successfully synthesized by the pre-graphenization route. The as-prepared products were characterized by various analyses such as: SEM, TEM, EDS, VSM, and XRD. The optical band gaps of the pure NiO and NiO/graphene nanocomposite were estimated about 3.3 eV, so these products were considered as semiconductors that can be used as photocatalyst. Increasing the surface and decreasing the possibility of (e-h +) recombination of nanocomposite were led to the better photocatalytic performance of NiO/graphene nanocomposite than pure NiO. According to the obtained results, degradation of methyl orange (MO) in the presence of NiO/graphene nanocomposite under UV and visible light as about 90.3% after 75 min, and 78% after 180 min, respectively.