Single event upset vulnerabilities of static random access memory (SRAM) cells on the micron scale and deep sub-micron scale are characterized and analyzed under ionizing irradiation. Meanwhile, by means of three-dimentional simulation, electrical responses of 6-T SRAM cell with feature size 0.18 μm are calculated when ions are injected into the different central single transistors under the irradiotion with different deposited doses. The simulation results are consistent with the analysis conclusion: the single event upset vulnerability would increase only when the SRAM cell stores the same state as the one stored in the irradiation period.