Provided are a semiconductor device having a buried word line structure in which a gate electrode and a word line may be buried within a substrate to reduce the height of the semiconductor device and to reduce the degradation of the oxide layer caused by chlorine ions from the application of a TiN metal gate, and a method of fabricating the semiconductor device. The semiconductor device may comprise a semiconductor substrate defined by a device isolation layer and comprising an active region including a trench and one or more recess channels, a gate isolation layer on the surface of the trench, a gate electrode layer on the surface of the gate isolation layer, and a word line by which the trench may be buried on the surface of the gate electrode layer.