Abstract: Advances in the manufacture of SRAMs over the years have resulted in enormous gains in speed, array size, cell density, and reduction in power consumption. ESA and NASA’s experts have studied the SEE response of these SRAMs, and discovered the extreme sensitivity to SEL in current generations of SRAM devices. Recently, we have tested on Samsung 4M SRAM using a pulsed laser system and a heavy ion accelerator. Heavy ion test shows that the latch-up LET threshold of Samsung 4M SRAM KM684002 is above 39.6 MeV·cm2/mg, but the new Rev. D Samsung 4M SRAM K6R4016C1D has a latch-up LET threshold of below 1.5 MeV?cm2/mg. If K6R4016C1D was used in spacecraft, 0.008～0.04/day SEL would happened, which is a great threat to our aerospace safety. So it should be warned in using these extreme SEL susceptibility devices.