The double snapback characteristic in the highvoltage nMOSFET under transmission line pulsing stress is found. The physical mechanism of double snapback phenomenon in the high-voltage nMOSFET is investigated by device simulation. With double snapback characteristic in high-voltage nMOSFET, the holding voltage of the high-voltage nMOSFET in snapback breakdown condition has been found to be much smaller than the power supply voltage. Such characteristic will cause the high-voltage CMOS ICs susceptible to the latchup-like danger in the real system applications, especially while the high-voltage nMOSFET is used in the power-rail electrostatic discharge clamp circuit.Index Terms-Double-diffused drain (DDD), electrostatic discharge (ESD), high-voltage nMOSFET, lateral diffused MOS (LDMOS), latchup.