The authors evaluated the local dielectric constant for the ultrathin beta-quartz (0001) films and the ideal quartz/Si(001) interface and studied the effect of oxygen vacancy on the local dielectric constant, using first-principles calculations in external electric fields. The optical dielectric constant in the SiO2 film is increased slightly around the oxygen vacancy, while the static dielectric constant is increased remarkably. These enhancements are caused by introduction of Si+3 state by the oxygen vacancy. For the ideal quartz/Si(001) interface, both the optical and the static dielectric constants change abruptly from each bulk value to another one. The oxygen vacancy located just at the interface layer changes the interface Si+2 state into Si+1 state and increases both the optical and the static dielectric constant in the adjacent Si region. (C) 2008 American Vacuum Society.