The total ionizing dose (TID) effects on 55 nm SONOS flash cell, caused by ⁶⁰Co-γ ray and 10 keV X-ray radiation source, are systematically investigated in this paper. The degradation of electrical characteristics is discussed while the underlying physical mechanism is analyzed. The drift of I-V characteristic curve, the degradation of memory window, and the increase of stand-by current are observed after TID irradiation separately by the two radiation sources. The data retention capability is also affected by the TID irradiation. The I-V_g curve of the programmed single flash cell significantly drifts towards the negative direction after TID irradiation, while the negative drift of erased state is much slower. Referring to the erased state, the drift directions of I_d-V_g curves for γ- and X-ray radiation source are obviously different. The physical mechanism of irradiation damage in a 55 nm SONOS single flash cell is discussed in detail by the energy band theory and TCAD simulations. The storage charge loss in silicon nitride layer, the charge accumulation, and the generation of interface states all together lead to the degradation of threshold voltage and stand-by current after TID irradiation. Another cause for the increase of stand-by current is the positive trapped charges in the isolated oxide induced by irradiation, which leads to the generation of leakage paths. Significant dose enhancement effect of X-ray irradiation is observed in this paper. Device model of memory transistor c is established while the dose enhancement effect of X-rays is investigated by Geant 4 tool. The high-Z materials above the poly-silicon gate lead to the dose enhancement effect of X-rays’ irradiation, which results in the higher degradation. The density of electron-hole pairs produced by irradiation in W layer is much higher than in Cu layer. In particular, W layer is a critical factor regardless of the thickness, which can be obviously observed in the simulation.