Latchup failure induced by electrostatic discharge (ESD) protection circuits occurred anomalously in a high-voltage IC product. Latchup issues existed at only three output pins, two of which belonged to the top and the other to the side. The layouts of top and bottom output pins are identical, and side output pins have another identical layouts. In our experiments it was found latchup of two top output pins were originated from the latchup of the side output pin, and therefore heat-induced latchup aggravation issue must be noticed during latchup test. Furthermore, large power line current (Idd) existed during triggering this side output pin and led to subsequent latchup. After thorough layout inspection, the layout of this side output pin is identical to all other side output pins except that it has an additional N-well (NW) resistor of gate-triggered high-voltage PMOS beside. It was later proved by engineering experiments that this NW resistor is the origin of inducing latchup in this product, and a new mechanism was proposed for this latchup failure. Improvements and solutions were also provided to successfully solve the latchup issues of these three output pins.