A high voltage metal-oxide-semiconductor (HV MOS) transistor device includes a substrate, a drifting region formed in the substrate, a plurality of isolation structures formed in the drift region and spaced apart from each other by the drift region, a plurality of doped islands respectively formed in the isolation structures, a gate formed on the substrate, and a source region and a drain region formed in the substrate at respective two sides of the gate. The gate covers a portion of each isolation structure. The drift region, the source region, and the drain region include a first conductivity type, the doped islands include a second conductivity type, and the first conductivity type and the second conductivity type are complementary to each other.