In a method of manufacturing a semiconductor device, first to third active fins are formed on a substrate. Each of the first to third active fins extends in a first direction, and the second active fin, the first active fin, and the third active fin are disposed in this order in a second direction crossing the first direction. The second active fin is removed using a first etching mask covering the first and third active fins. The third active fin is removed using a second etching mask covering the first active fin and a portion of the substrate from which the second active fin is removed. A first gate structure is formed on the first active fin. A first source/drain layer is formed on a portion of the first active fin adjacent the first gate structure.