A semiconductor device includes active fins on a substrate, a first isolation pattern on the substrate, the first isolation pattern extending on a lower sidewall of each of the active fins, a third isolation pattern including an upper portion extending into the first isolation pattern and a lower portion extending into an upper portion of the substrate, the lower portion contacting the upper portion of the third isolation pattern, and having a lower surface with a width greater than that of an upper surface thereof, and a second isolation pattern extending in the substrate under the third isolation pattern, contacting the third isolation pattern, and having a rounded lower surface.