A semiconductor device includes: a gate trench formed into a semiconductor substrate; a gate dielectric layer formed in the gate trench to cover an inside surface of the gate trench; and a gate electrode disposed over the gate dielectric layer to fill the gate trench, wherein the gate electrode includes: second crystal grains formed in the gate trench; and first crystal grains disposed between the second crystal grains and the gate dielectric layer and having a smaller crystal grain size than the second crystal grains.