It has been long suggested that the ≡SiOH defect is an electron or "water" trap in silicon dioxide based on some indirect evidences. In this work, quantum calculation on the capturing properties of non-bridging oxygen hole center with unpaired electron ≡SiO^· and hydrogen defect ≡SiOH in silicon oxide are performed with the ab initio density-functional method. It was found that the ≡SiO^· defect is an electron trap and this defect should be the responsible candidate for better hardness against radiation for the metal-oxide-semiconductor gate oxide produced by wet oxidation. We found that the ≡SiOH defect could not be an electron trap according to the present calculation results.