Nickel oxide was prepared by modified sol–gel method and spherical NiO nanoparticles were obtained. The factors that influence the physical property of NiO nanoparticles were investigated mainly through the use of SEM, BET and XRD. It revealed that calcination temperature of the precursor and the pH value of the solution significantly affected the particle size and surface area of the synthesized nanosized NiO powders. The ratio of citric acid to nickel nitrate and the heating rate of calcinations were also important to the physical property of nanoscaled NiO. The length of calcination time has a minor effect on it.