An electrostatic discharge (ESD) protection circuit is provided. The ESD protection circuit includes a first NMOS transistor coupled to a power line, a second NMOS transistor coupled between the first transistor and a ground, a detection unit, providing a detection signal when an ESD event occurs at the power line, and a trigger unit, turning on the second NMOS transistor and the first NMOS transistor in sequence in response to the detection signal, such that a discharge path is formed from the power line to the ground via the first and second NMOS transistors. The trigger unit includes a first PMOS transistor coupled between the power line and a gate of the second NMOS transistor, a fourth resistor, and a second PMOS transistor, having a gate coupled to the cathode of the diode for receiving the detection signal.