A fabrication method of a trenched power MOSFET is provided. A pattern layer having a first opening is formed on a substrate. A portion of the substrate is removed, using the pattern layer as a mask, to form a trench in the substrate. A width of the trench is expanded. A gate oxide layer is formed on a surface of the trench. A portion of the gate oxide layer on a bottom of the trench is removed, using the pattern layer as a mask, to form a second opening in the gate oxide layer. The width of the expanded trench is greater than that of the second opening. A thick oxide layer is formed in the second opening. Heavily doped regions are formed beside the thick oxide layer. A gate is formed in the trench. A body layer surrounding the trench is formed. Sources are formed beside the trench.