The effect of oxygen pressure on the initial sintering of nickel oxide has been investigated by measuring the isothermal shrinkage of compacts at 690° to 780°C in the oxygen pressure range of 10-11-1 atm. The kinetic data were analyzed with an equation for initial combined sintering of surface and volume diffusions. The oxygen partial pressure (Po2) was controlled by using CO2-O2 and CO2-CO mixtures and measured by means of a solid electrolyts cell (Ca stabilized ZrO2). The results indicated that the volume diffusion coefficient was proportional to Po21/4-1/6 in the pressure range of Po2>10-3 atm and almost independent of the pressure of Po2<10-3 atm. This range was experimentally found to agree with the impurity (monovalent) range of nickel oxide. The surface diffusion coefficient, in general, increased with Po2 but the quantitative relation was not obtained. In this experiment, the sintering of nickel oxide was concluded to be controlled by the diffusion of trivalent nickel ions (Ni3+) partially associated with cation vacancies.