Provided are methods of fabricating a semiconductor device including a field effect transistor. Such methods may include sequentially forming lower and intermediate mold layers on a substrate, forming first upper mold patterns and first spacers on the first and second regions, respectively, of the substrate, etching the intermediate mold layer using the first upper mold patterns and the first spacers as an etch mask to form first and second intermediate mold patterns, respectively, forming second spacers to cover sidewalls of the first and second intermediate mold patterns, etching the lower mold layer using the second spacers as an etch mask to form lower mold patterns, and etching the substrate using the lower mold patterns as an etch mask to form active patterns.