Misunderstanding apparent activation energy (E_aa) can cause serious error in lifetime predictions. In this paper, the E_aa is investigated for sub 20 nm NAND flash memory. In a high-temperature (HT) regime, the interface trap (N_it) recovery mechanism has the greatest impact on the charge loss. However, the values of E_aa and E_a(Nit) have a wide difference. Also, the lifetime of the device cannot be estimated by the Arrhenius model due to the E_aa roll-off behavior. For the first time, we reveal the origin of abnormal characteristics on E_aa and derive a mathematical formula for E_aa as a function of each E_a(mechanism) in NAND flash memory. Using the proposed E_aa equation, the accurate lifetime for the device is estimated.