The disclosure discloses a structure of high-voltage (HV) transistor which includes a substrate. An epitaxial doped structure with a first conductive type is formed in the substrate, wherein a top portion of the epitaxial doped structure includes a top undoped epitaxial layer. A gate structure is disposed on the substrate and at least overlapping with the top undoped epitaxial layer. A source/drain (S/D) region with a second conductive type is formed in the epitaxial doped structure at a side of the gate structure. The first conductive type is different from the second conductive type.