A HVMOS transistor device is provided. The HVMOS has a substrate, a gate structure, a drain region and a source region, a base region and a gate dielectric layer. The substrate has a first insulating structure disposed therein. The gate structure is disposed on the substrate and comprises a first portion covering a portion of the first insulating structure. The drain region and the source region are disposed in the substrate at two respective sides of the gate, and comprise a first conductivity type. The base region encompasses the source region, wherein the base region comprises a second conductivity type complementary to the first conductivity type. The gate dielectric layer is between the gate and the drain region, the base region and the substrate. The gate structure further comprises a second portion penetrating into the base region. A method of forming the HVMOS is further provided.