We studied the dielectric properties of ultrathin SiO2 films using two methods, internal field method and dipole moment method, based on first-principles calculations of the ground state in a finite electric field. We evaluated the optical and static dielectric constants, epsilon(infinity) and epsilon(0), of H-terminated beta-quartz (0001) films through calculations without and with the lattice relaxation in the electric field applied, respectively. The calculated values of E. and so with these two methods are in good agreement with each other, and they reproduce well the experimental values. Both the optical and static dielectric constants hardly depend on the film thickness, and the spatial variation of the local dielectric constant is also very small. These results indicate that both the surface effect and the quantum confinement effect are small for the ultrathin H-terminated beta-quartz (0001) films. (c) 2006 American Vacuum Society.