A semiconductor device includes a substrate including an active pattern; a channel pattern on the active pattern, the channel pattern including a plurality of semiconductor patterns, which are stacked to be spaced apart from each other; a source/drain pattern connected to the plurality of semiconductor patterns; a gate electrode on the plurality of semiconductor patterns; and a blocking layer between the source/drain pattern and the active pattern, wherein the source/drain pattern includes a protruding side surface protruding toward the semiconductor patterns, the blocking layer includes silicon-germanium (SiGe), and a germanium concentration of the blocking layer is higher than a germanium concentration of the source/drain pattern.