A semiconductor device includes a plurality of active fins defined by an isolation layer on a substrate, a gate structure on the active fins and the isolation layer, and a gate spacer structure covering a sidewall of the gate structure. A sidewall of the gate structure includes first, second, and third regions having first, second, and third slopes, respectively. The second slope increases from a bottom toward a top of the second region. The second slope has a value at the bottom of the second region less than the first slope. The third slope is greater than the second slope.