A semiconductor device includes first and second active patterns extending in a first direction, a first epitaxial pattern on the first active pattern and adjacent to the second active pattern, a second epitaxial pattern on the second active pattern and adjacent to the first active pattern, an element separation structure separating the first and second active patterns between the first and second epitaxial patterns, and including a core separation pattern, and a separation side wall pattern on a side wall of the core separation pattern, and a gate structure extending in a second direction intersecting the first direction, on the first active pattern. An upper surface of the gate structure is on the same plane as an upper surface of the core separation pattern. The separation side wall pattern includes a high dielectric constant liner, which includes a high dielectric constant dielectric film including a metal.