An elementary step of trap and detrap processes of electron in the tunnel oxide during program/erase in NAND Flash memory is precisely studied. Owing to the high electric field during program and erase (P/E), the electron trapping and detrapping occur at the same time. Consequently, the detrapping process only leaves electrons in the deeper trap energy state (Etrap) than 3.5 eV. In addition, as-grown trap density (Ne) and capture cross section (σ) in the deep trap state can be specified to explain the measured data including the tunneling current modulation with cycling and the VT shift by oxide trap. The P/E endurance characteristics using dry and plasma oxidation processes are analyzed and compared. In both processes, σ has the same value (4×10-17 cm2). However, Ne depends on the oxidation process. In dry oxidation, Ne is 1.88×1019 cm-3. On the other hand, in plasma oxidation, 30% reduction (1.25×1019 cm-3) can be found.