The influences of total ionizing dose (TID) on the single event effect (SEE) sensitivity of 34-nm and 25-nm NAND flash memories are investigated in this paper. The increase in the cross section of heavy-ion single event upset (SEU) in memories that have ever been exposed to TID is observed, which is attributed to the combination of the threshold voltage shifts induced byγ-rays and heavy ions. Retention errors in floating gate (FG) cells after heavy ion irradiation are observed. Moreover, the cross section of retention error increases if the memory has ever been exposed to TID. This effect is more evident at a low linear energy transfer (LET) value. The underlying mechanism is identified as the combination of the defects induced byγ-rays and heavy ions, which increases the possibility to constitute a multi-trap assisted tunneling (m-TAT) path across the tunnel oxide.