A fabrication method of a trenched metal-oxide-semiconductor device is provided. Firstly, an epitaxial layer is formed on a substrate. Then, a plurality of gate trenches is formed in the epitaxial layer. Afterward, a spacer is formed on the sidewall of the trench gates. The spacer is utilized as a mask to selectively implant oxygen ion into the bottom of the gate trenches so as to form a bottom oxide layer on the bottom of the gate trenches to reduce capacitance between gate and drain.