X-ray techniques were employed to characterize the interfacial region in the Si-on-sapphire heterostructure. Strained interfacial layer is formed at the top of the sapphire substrate. Application of the biaxial strain model to the strain-stress relation transformed to the actual crystal system attached to the sapphire surface (01 (1) over bar2) allowed to derive the strain/stress tenser at the interface. In this mismatched heterostructure the wafer aquires an overall curvature and the elastic anisotropicity, in particular that of the sapphire substrate, has a detrimental effect on the surface topography of the Si epilayers. The bending profile visualized in the laser light interference patterns reflects the strain/stress distribution determined for the interfacial layer.