An extraction method for single event upset cross section based on Monte Carlo code and device simulation is proposed, which can be used to calculate single event effects and sensitive regions in memories accurately. Single event upset cross sections of domestic static random access memory (SRAM) and field programmatic gate array (FPGA) devices are calculated, and results agree well with these from heavy ion test. Simulation results reveal the physical mechanism of the relationship between single event upset sensitivity and surface area of off-state NMOSFET and PMOSFET. Sensitive regions of single event upset under different linear energy transfer (LET) values are obtained. The radial ionization profiles of heavy ions with different energy, but the same LET, are also calculated using the Monte Carlo method. The track radius of high-energy ion is significantly larger than that of low-energy ion, while the charge density at the track center of low-energy ion is higher by two or three orders of magnitude. With decreasing technology scaling, the impact of these differences on single event effects will be more pronounced, and the threshold LET and saturated cross-section will not be capable of describing the single event response completely.