A method of fabricating a semiconductor device having a vertical channel transistor, the method including forming a hard mask pattern on a substrate, forming a preliminary active pillar by etching the substrate using the hard mask pattern as an etch mask, reducing a width of the preliminary active pillar to form an active pillar having a width less than that of the hard mask pattern, forming a lower source/drain region by implanting impurity ions into the substrate adjacent to the active pillar using the hard mask pattern as an ion implantation mask, and forming an upper source/drain region on the active pillar and vertically separated from the lower source/drain region.