A HV MOS transistor device having a substrate, a gate, a source, a drain, a first ion well of a first conductive type disposed in the substrate, and a plurality of field plates disposed on the substrate is disclosed. The HV MOS transistor device further has a first doped region of a second conductive type positioned in the first ion well. Therefore, a first interface and a second interface between the first ion well and the first doped region are formed, and the first interface and the second interface are respectively positioned near the drain and the source. In addition, the first interface is positioned under a respective field plate to produce a smooth field distribution and to increase the breakdown voltage of the HV transistor device.