In this paper, we propose a novel thin gate-oxide-thickness measurement method by using Lightly-Doped-Drain (LDD) N-Channel Metal-Oxide-Semiconductor (NMOS) snapback characteristics. For device with thin oxide 150 Å~60 Å and optimized LDD doping profile, the drain breakdown voltage will be oxide-thickness limited, and therefore the oxide breakdown's critical-field is about 10 MV/cm. By this characteristic, a high-voltage sine-waveform generator provides the necessary voltage to lead to the grounded-gate LDD-NMOS happening gate-assisted drain breakdown and drives its parasitic n-p-n bipolar turn-on. The voltage-waveform collapses immediately as the applied-voltage exceeds the critical oxide-field 10 MV/cm. Therefore, the oxide thickness can be determined.