Total dose irradiation and the hot-carrier damages are two of the important factors for the application of sub-micro and even smaller MOS devices. Therefore, how to prevent the device from being damaged attracts much attention. Total dose irradiation and hot-carrier effects of sub-micro NMOSFET with various channel sizes are studied. Electronic parameters are measured and the results show that though the principles of damages are somewhat similar, the total dose irradiation and the damage behavior and their dependences on the width-to-length(W/L) ratio of channel size for these two effects are different. The most notable damage of radiation lies in the great increase of the off-state leakage current, and the damage increases withW/L reducing. While for hot-carrier effect, several parameters such as trans-conductance change a lot, except for the off-state leakage current. And the damage increases as channel length and channel width decrease. The different damage behaviors and different relations to channel size are attributed to the different location of charges induced. Therefore, different aspects should be considered when the device is hardened against these two effects.