The present study deals with thermo-mechanical behaviour of aluminium oxide single crystals under thermal stress. In the experiments the (0001), (1120) and (1102) faces are tested under pulse heating in plasma of varying intensity. The peak temperature on the sample surface is determined by measuring the separation between fragments formed by cracks during the cooling stage of the heating-cooling cycle. The fracture morphology and fracture threshold are analysed experimentally and theoretically on all three crystal faces tested in plasma. A theoretical model is based on the continuum mechanics approach which takes into account anisotropy of elastic, plastic and thermal properties of sapphire crystals. The thermal stress resistance parameter is theoretically analysed for the faces under investigation. The parameter allows one to determine the resistance to fracture and plastic deformation for various crystal faces in sapphire and thus to identify the most resistant crystal face under thermal shock loading. The theoretical results on fracture morphology and fracture threshold proved to be consistent with the experimental ones. The (1102) face in sapphire crystals is demonstrated to be most resistant to fracture, and the (0001) face most resistant to plastic deformation under the plane state of stress applied. The model is shown to be suitable for determining the temperature threshold of fracture and plastic deformation and predicting the fracture morphology in brittle crystals with various faces.