The electronic structure of SOI (silicon-on-insulator) with strained and unstrained silicon layers was theoretically calculated and experimentally investigated by means of Ultrasoft X-ray Emission and Absorption Spectroscopy. According to the experimental results, an additional spectral feature occurs in the strained silicon layer density of states near the bottom of the conduction band (E-c). The shift of E-c towards the top of the valence band, as well as smoothing of the density of states and disappearance of the degenerate minimum between L'(2v) and L-1v valence band states was also observed. The theoretical calculations have been performed by full potential linearized augmented plane-wave method and show that straining of silicon lattice leads to a slight shifting of the conduction band bottom towards the top of the valence band and causes an increase in the density of states between L'2 and Li,. The theoretical shift of the conduction band bottom is substantially less than the experimental one. (C) 2016 Elsevier B.V. All rights reserved.