Methods for fabricating semiconductor devices are provided including sequentially stacking hardmask layers, a first sacrificial layer, and a second sacrificial layer on a substrate, forming first mandrels on the first sacrificial layer by etching the second sacrificial layer, forming first spacers on side walls of the first mandrels, forming a photoresist pattern disposed outside a region from which the first mandrels have been removed, forming second and third mandrels by etching the first sacrificial layer using the first spacers and the photoresist pattern as respective etching masks, forming second and third spacers on side walls of the second and third mandrels, forming first and second active patterns respectively having first and second pitches by etching the hardmask layer and at least a portion of the substrate, and forming a device isolation layer so that upper portions of the first and second active patterns protrude therefrom.