A vertical channel transistor in semiconductor device and method of fabricating the same are disclosed. A method of fabricating a vertical channel transistor for a semiconductor device includes forming, on a substrate, a plurality of active pillars each having a gate electrode formed on and surrounding a lower portion thereof; forming a first insulation layer over the active pillars to fill a gapregion between the active pillars; partially removing the first insulation layer to exposes a circumferential surface of the gate electrode in all directions, without exposing the substrate in the gap region between the active pillars; forming a conductive layer on the remaining first insulation layer to fill the gap region between the active pillars; and patterning the conductive layer to form aword line that surrounds and contacts the circumferential surface of the gate electrode in all directions.