The characteristics of an Al2O3 coating on a Nb or Mo substrate using a direct current-radio frequency induction (DC-RF) thermal plasma of Ar-H-2 were studied. A porous and crystallographically stable coating with highly preferred orientation of {110} planes of alpha-Al2O3 crystals was obtained. The preferred orientation was strongly dependent on the diameter of the introduced Al2O3 powder. The formation mechanism of the preferred orientation of the coating is explained by accumulation of charged clusters on the substrate. The coating prepared by this procedure revealed the superior capability of protecting the substrate against high-temperature oxidation.