The conductivity of polycrystalline NiO is measured in the temperature range of $200^{circ}C;to;800^{circ}C$ under oxygen pressures from $1.52{ imes}10^2;mmHg;to;10^{-4}$ mmHg. The plots of the log ${sigma}$ vs 1/T at constant oxygen pressure are found to be linear and the activation energies obtained from the slopes of these plots show that the energies are greater under high oxygen pressure than under low pressure. The transition points are found from the curves. The dependence of the conductivity on the $O_2$ pressure, in the above temperature range, is to be regular but it does not obey the theoretical expression, i.e. ${sigma}=K_{ox}P^{1/6}.$ The activation energies are calculated from the curves at the various condition.