A semiconductor device is provided as follows. Active fins protrude from a substrate, extending in a first direction. A first device isolation layer is disposed at a first side of the active fins. A second device isolation layer is disposed at a second side of the active fins. A top surface of the second device isolation layer is higher than a top surface of the first device isolation layer and the second side is opposite to the first side. A normal gate extends across the active fins in a second direction crossing the first direction. A first dummy gate extends across the active fins and the first device isolation layer in the second direction. A second dummy gate extends across the second device isolation layer in the second direction.