Abstract: Semiconductor device and method for fabricating the same, in which a gate electrode is formed buried in a surface of a substrate, for improving device performance. The device includes device isolation layers each buried in a device isolation region of a semiconductor substrate for defining active regions, first gate trenches each with a generally square section formed in a surface of the active region, and second gate trenches each with an elliptical section formed in continuation from the first gate trench. A gate oxide film is formed on surfaces of the first and second gate trenches. Gate electrodes are formed in the first and second trenches having the gate oxide film formed thereon, and source/drain regions are formed in surfaces of the semiconductor substrate on both sides of the gate electrodes insulated from the gate electrodes by the gate oxide film.