In order to make clear the effect of strain and intrinsic defects in SiO2 on both its electronic and structural characteristics, we performed a quantum chemical molecular dynamics analysis for SiO2-x structure under strain. The band gap of SiO2 changes significantly by the deformation of the Si-O-Si bond angle due to the applied strain. The cristallographic structure of the SiO2-x deforms drastically because the Si-O bonds neighboring an oxygen vacancy are broken and a free silicon monoxide molecule is generated in the SiO2-x structure. The magnitude of the band gap of the SiO2-x decreases from 8.9 eV to 6.3 eV due to the change in the atomic configuration accompanying the diffusion of the free monoxide. In addition, the band gap decreases further under large tensile strain. 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.