A method of fabricating a semiconductor device includes patterning a substrate to form an active fin, forming a sacrificial gate pattern crossing over the active fin on the substrate, removing the sacrificial gate pattern to form a gap region exposing the active fin, and forming a separation region in the active fin exposed by the gap region. Forming the separation region includes forming an oxide layer in the exposed active fin and forming an impurity regions with impurities implanted into the exposed active fin.