We performed a quantum chemical molecular dynamics analysis for SiO2-x structure under strain to make clear the effect of the strain and intrinsic defects on both electronic and structural characteristics of SiO2-x. The SiO2-x showed a large change of the structure during the simulation. This is mainly because that the Si-O bonds near an oxygen vacancy were broken and a free silicon monoxide molecule was generated in the SiO2-x structure. The magnitude of the band gap of the SiO2-x decreased drastically due to the formation of the free monoxide. In addition, the band gap decreased further under large tensile strain of about 10%. We can conclude therefore, that both the existence of oxygen vacancies and tensile strain in SiO2-x films deteriorate the electronic reliability of the oxide film seriously.