5 nm-thick SiO2 layers formed by plasma-enhanced chemical vapor deposition (PECVD) are densified to improve the electrical and interface properties by using nitric acid oxidation of Si (NAOS) method at a low temperature of 121 oC. The physical and electrical properties are clearly investigated according to NAOS times and post-metallization annealing (PMA) at 250 oC for 10 min in 5 vol% hydrogen atmosphere. The leakage current density is significantly decreased about three orders of magnitude from 3.110 × 10−5 A/cm2 after NAOS 5 hours with PMA treatment, although the SiO2 layers are not changed. These dramatically decreases of leakage current density are resulted from improvement of the interface properties. Concentration of suboxide species (Si1+, Si2+ and Si3+) in SiOx transition layers as well as the interface state density (Dit) in SiO2/Si interface region are critically decreased about 1/3 and one order of magnitude, respectively. The decrease in leakage current density is attributed to improvement of interface properties though chemical method of NAOS with PMA treatment which can perform the oxidation and remove the OH species and dangling bond.