A semiconductor device includes first and second active patterns on a substrate, the first and second active patterns adjacent to each other in a first direction with a first trench between the first and second active patterns, third and fourth active patterns on the substrate, the third and fourth active patterns adjacent to each other in the first direction with a second trench between the third and fourth active patterns. The semiconductor device includes a first device isolation layer in the first trench, and a second device isolation layer in the second trench. A width of the second trench in the first direction is greater than a width of the first trench in the first direction. The second device isolation layer includes a first protrusion and a second protrusion which protrude from a top surface of the second device isolation layer.