The structures of films of 300Å‐15μ thick formed on (100), (110), and (111)Ni crystal faces at temperatures in the range 500°–800°C were investigated by electron microscopy and x‐ray diffraction. The oxide was initially polycrystalline but rearranged as the oxide thickened to major epitaxial relationships with the metal substrate. These relationships were characterized by coincidence between the close‐packed directions of the metal and oxide; the number and type of oxide orientations were correlated with the number of close‐packed directions in the metal faces. Oxide thicker than 1μ exhibited preferred {100} and {111} orientations. The oxide on (100) and (111)Ni exhibited the highest preferred orientation and a duplex layer structure consisting of equiaxed crystallites in the inner layer and columnar ones in the outer layer. These findings are rationalized in terms of a diffusional model for the growth of these oxide layers in which the reaction rate is determined by boundary and lattice diffusion of nickel through nickel oxide.