Abstract: By hydrothermal method combining a calcination process, the NiO nano-catalysts with different morphologies and sizes were synthesized. The properties of NiO samples were characterized by SEM, XRD, TG and UV-vis analysis. In the photodegradation of methylene blue (MB) under UV-light irradiation on NiO surface, the result showed that the NiO sample which was synthesized by the reaction of nickel nitrate with sodium hydroxide and calcination at 6000C exhibits the highest photocatalytic activity in all of as-prepared NiO samples. This is because the NiO has a small particle size, large surface area, and intense UV absorption, inducing the increase of hydroxy radical OH? and the enhance of degradation efficiency.