A novel MOS transistor, which includes a source region, a drain region, a channel region, an isolation region, a drift region, a gate dielectric layer, a gate electrode and a field plate, is provided. The gate electrode has a first portion and a second portion. The first portion of a first conductivity type is located over the channel region and has a width equal to or greater than a distance of the gate electrode overlapped with the channel region. The second portion is un-doped and located over the isolation region. Accordingly, the MOS transistor allows higher process freedom saves production cost, as well as improves reliability.