In order to quickly discharge the electrostatic discharge (ESD) energy, an unassisted low-voltage-trigger ESD protection structure is proposed in this work. Under transmission line pulsing (TLP) stress, the trigger voltage, turn-on speed and second breakdown current can be obviously improved, as compared with the traditional protection structure. Moreover there is no need to add any extra mask or do any process modification for the new structure. The proposed structure has been verified in foundry's 0.18-µm CMOS process.