A high voltage metal-oxide-semiconductor transistor device includes a substrate having an insulating region formed therein, a gate covering a portion of the insulating region and formed on the substrate, a source region and a drain region formed at respective sides of the gate in the substrate, a body region formed in the substrate and partially overlapped by the gate, and a first implant region formed in the substrate underneath the gate and adjacent to the body region. The substrate and body region include a first conductivity type. The source region, the drain region, and the first implant region include a second conductivity type. The first conductivity type and the second conductivity type are complementary to each other.