A fabricating method of a trench-gate metal oxide semiconductor device is provided. The fabricating method includes the steps of defining a first zone and a second zone in a substrate, forming at least one first trench in the second zone, forming a dielectric layer on the first zone and the second zone, filling the dielectric layer in the first trench, performing an etching process to form at least one second trench in the first zone by using the dielectric layer as an etching mask, forming a first gate dielectric layer on a sidewall of the second trench, and filling a conducting material layer into the second trench, thereby forming a first gate electrode.