A semiconductor device includes a substrate including an active fin structure, a plurality of gate structures, a first spacer on sidewalls of each of the gate structures, and a second spacer on sidewalls of the first spacer. The active fin structure may extend in a first direction and including a plurality of active fins with adjacent active fins divided by a recess. Each of the plurality of gate structures may extend in a second direction crossing the first direction, and may cover the active fins. The first spacer may include silicon oxycarbonitride (SiOCN), and may have a first carbon concentration. The second spacer may include SiOCN and may have a second carbon concentration which is different from the first carbon concentration. The semiconductor device may have a low parasitic capacitance and good electrical characteristics.