A semiconductor device includes a substrate having first and second active regions, first and second active patterns on the first and second active regions, first and second gate electrodes running across the first and second active patterns, and a high-k dielectric layer between the first active pattern and the first gate electrode and between the second active pattern and the second gate electrode. The first gate electrode includes a work function metal pattern and an electrode pattern. The second gate electrode includes a first work function metal pattern, a second work function metal pattern, and an electrode pattern. The first work function metal pattern contains the same impurity as that of the high-k dielectric layer. An impurity concentration of the first work function metal pattern of the second gate electrode is greater than that of the work function metal pattern of the first gate electrode.