There is provided a semiconductor device capable of improving electrical characteristics and integration density. The semiconductor device includes an active pattern protruding from a substrate, the active pattern including long sidewalls extending in a first direction and opposite to each other in a second direction, a lower epitaxial pattern on the substrate and covering a part of the active pattern, a gate electrode on the lower epitaxial pattern and extending along the long sidewalls of the active pattern, and an upper epitaxial pattern on the active pattern and connected to an upper surface of the active pattern. The active pattern includes short sidewalls connecting with the long sidewalls of the active pattern, and at least one of the short sidewalls of the active pattern has a curved surface.