The oxidation of cold‐worked and annealed pure Ni in 1 atm O2 was investigated from 700° to 1270°C in continuous weighing experiments for periods from 1 min to 20 hr and the oxidized specimens examined by diffraction, electron‐optical techniques, and metallographic cross sections. Cold‐worked Ni was found to oxidize faster than annealed Ni and form finer‐grained oxide. Plots of the apparent parabolic rate constant, Kp, for cold‐worked Ni show an initial high value that decreases rapidly as the oxide coarsens with time. For annealed Ni, Kp is lower, grain size larger, and both change little with time. Accordingly, a range of Kp values is obtained at each temperature and, on an Arrhenius plot of Ni oxidation, there is a corresponding variation in apparent activation energy, EA. These results can be plausibly interpreted on the basis of oxide grain boundaries acting as easy diffusion paths for Ni through the layer: for fine‐grained oxide, Kp is higher and EA lower. The oxide is thinnest (and the derived EA highest) on Ni grains which form a single crystal overgrowth. A break in the Arrhenius plot around 1000°C is the result of transport below 1000° being largely by leakage paths while at higher temperatures volume transport is more important. The estimated activation energy for growth of the oxide layer is for transport by lattice diffusion and appreciably less for transport by grain boundary diffusion.