When the Si/SiO₂ interface is formed by thermal oxidation of Si, quite a large strain is induced by the volume expansion from Si to SiO₂ (approximately 2.25 times). This strain should have a great effect on the formation of the Si/SiO₂ interface. Here, we review our recent theoretical studies of the strain effect on the atomical flatness and electrical properties of the interface by using the first-principles calculation method. We also show that the Si emission at the interface is an important mechanism for releasing the strain.