The study of geometric effect on the electric property damage and degradation of 8-shape NMOS under TID (Total Ionizing Dose) radiation, and the comparison of immunity to which between standard and 8-shape NMOS with parameter variation differences were presented. As results showed, compared with standard NMOS, the influence of gate-active region overlaid width and aspect ratio on the 8-shape NMOS off-state current could be ignored. Drain saturation current of 8-shape NMOS with any geometric change, compared with standard NMOS, showed a better ability to be stable, regardless of TID levels. Meanwhile, differences of drain saturation current under small aspect ratio circumstance came from the variation of gate-active region overlaid width, which no longer existed as a contributor to the drain saturation current differences of 8-shape NMOS with big aspect ratio. Threshold voltage of all tested 8-shape NMOS fabricated in 180 nm process stayed at 0.41 V stably, superior to standard NMOS.