The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open‐space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho‐Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open‐space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.