A lateral double-diffused metal-oxide-semiconductor transistor device includes a substrate having at least a shallow trench isolation formed therein, an epitaxial layer encompassing the STI in the substrate, a gate, and a drain region and a source region formed in the substrate at respective two sides of the gate. The epitaxial layer, the source region and the drain region include a first conductivity type. The gate includes a first portion formed on the substrate and a second portion extending into the STI.