The oxidation of high purity polycrystalline nickel was studied at 700 degreesC under flowing oxygen. For 1 mum thick oxide scale, the microstructure is cellular. For 3 to 30 mum thick oxide scales, rounded oxide platelets are observed on the scale surface. There is a large effect of substrate orientation. The scale is duplex, i.e. composed of an external scale of columnar grains and an inner subscale of equiaxed microstructure. The ratio external scale thickness/internal scale thickness is constant (congruent to 2) and does not depend on total oxide scale thickness. The interface between internal scale and external scale corresponds to the initial surface of the metal. To explain these microstructural features, the mechanisms of scale growth proposed in the literature are critically reviewed. The relative position of the duplex oxide scale and the initial surface of the metal, and the relative growth of the outer and inner scales permits to conclude that NiO growth does not proceed only by the outward diffusion of cationic species in a dense scale. The duplex scale formation seems to be associated with the growth of the inner scale by inward transport of oxygen.