To be utilized in the high density dynamic random access memory devices, low-pressure dry oxidation of the very thin nitrides has been performed to successfully obtain the ultrathin capacitors’ dielectrics with the effective oxide thickness (t(ox,eff)) of 30 - 60 Angstrom. For the electrical properties, the low-pressure dry oxidized (LPDO) nitrides express thinner effective oxide thickness, lower leakage current, and higher reliability than do the atmospheric pressure dry oxidized (APDO) nitrides. After analyzing the diluted HF etching profiles, Auger electron spectrum and Fourier transform infrared spectrum data, an oxide/nitride/oxide (O/N/O) structure is realized for the LPDO samples with respect to the nitride/oxide structure for the APDO ones. This ultrathin O/N/O structure is believed to be the main cause of improving the electrical characteristics. Finally, a low-pressure enhanced oxidation model is also proposed to explain the unusual oxidation phenomenon on the top of the nitride films. The mechanisms of APDO and atmospheric pressure wet oxidation of nitride films are also compared.