The disclosed subject matter provides an LDMOS device and fabrication method thereof. In an LDMOS device, a drift region and a body region are formed in a substrate. A first trench is formed in the drift region and in the substrate between the drift region and the body region. The first trench is separated from the drift region by a first shallow trench isolation structure. A gate dielectric layer is formed on a side surface and a bottom surface of the first trench. A gate electrode filling up the first trench is formed on the gate dielectric layer with a top surface above a top surface of the semiconductor substrate. A source region is formed in the body region on one side of the gate electrode and a drain region is formed in the drift region on another side of the gate electrode.