Abstract: An electrostatic discharge protection circuit is used in an integrated circuit with a first sub-circuit working with a first working voltage source and a second sub-circuit working with a second working voltage source lower than the first working voltage source. The electrostatic discharge protection circuit includes a first metal-oxide-semiconductor transistor of a first conductive type, having a drain thereof electrically connected to a pad of the integrated circuit, and gate, source and bulk thereof electrically connected to a bulk voltage; and a guard ring of the first conductive type, surrounding the first metal-oxide-semiconductor transistor of the first conductive type and coupled to the second working voltage source.