The average growth stress of NiO scale formed on pure Ni at 1000 degreesC in air was investigated by a modified deflection technique. The growth stress in NiO scale was tensile with a magnitude of about 10 MPa during the 10-h oxidation period. The growth stress level decreased with increasing oxidation time or oxide thickness. It varied from 37 MPa for a scale thickness of 4.8 mum to 13 MPa for a thickness of 13.8 mum. At the later stage of oxidation, the growth stress did not change noticeably. The planar stress state in the substrate was both compressive and tensile during the first hour of the deflection test. After that, only compressive stress existed in the substrate alloy. Published by Elsevier Ltd.