A method of forming a HVMOS transistor device is provided. A substrate is provided. A first insulation structure and a trench are formed in the substrate. A base region having a second conductivity type is formed, wherein the base region completely encompasses the trench. Next, a gate dielectric layer and a gate structure are formed in the trench and covering a portion of the first insulation structure. Then, a drain region and a source region are formed in the substrate at two respective sides of the gate structure, and the drain region and the source region comprise a first conductivity type complementary to the second conductivity type. A channel is defined between the source region and the drain region along a first direction.