An electrostatic discharge (ESD) protection device includes a first transistor and a second transistor. The first transistor includes a first bulk electrode, a first electrode and a second electrode. The first bulk electrode and the first electrode form a first parasitic diode. The first bulk electrode and the second electrode form a second parasitic diode. The second transistor includes a second bulk electrode, a third electrode and a fourth electrode. The second bulk electrode and the third electrode form a third parasitic diode. The second bulk electrode and the fourth electrode form a fourth parasitic diode. The first bulk electrode is connected to the third electrode, and the second bulk electrode is connected to the first electrode.