A fabrication method of trenched metal-oxide-semiconductor device is provided. A pattern layer with a plurality of openings is formed on a semiconductor base, and then a spacer is formed on the sidewall of the opening to define the gate trench. After the gate electrode formed in the gate trench, a dielectric structure is formed on the gate electrode by filling dielectric material into the opening. Then, the pattern layer and the spacer are removed and a dielectric layer is formed on the dielectric structure. The portion of the dielectric layer on the sidewall of the dielectric structure defines the source regions. After the source regions are formed in the well, another dielectric layer is formed on the dielectric layer to define the heavily doped regions adjacent to the source regions.