A method of manufacturing a semiconductor device may include forming first trenches that define active patterns extending in a first direction on a substrate, forming first insulating layers filling the first trenches, forming first mask patterns extending in the first direction while having a first width along a second direction perpendicular to the first direction, forming a second mask pattern extending in the first direction while having a second width along the second direction, and forming a second trench that partly defines an active region by executing a first etching process that etches the active patterns and the first insulating layer using the first mask patterns and the second mask pattern.