Electrostatic discharge protection circuit

An ESD protection circuit including a substrate of a first conductivity type, an annular well region of a second conductivity type, two first regions of the first conductivity type and at least one transistor of the second conductivity type is provided. The annular well region is disposed in the substrate. The first regions are disposed in the substrate and surrounded by the annular well region. The at least one transistor is disposed on the substrate between the first regions and including a source, a gate, and a drain. The annular well region and the drain are coupled to a first voltage source. The source and one of the first regions are coupled to a second voltage source, and the other of the first regions is coupled to a substrate triggering circuit.