A semiconductor device includes a substrate, a first gate structure and a second gate structure, a first gate spacer and a second gate spacer. The first gate spacer includes a first layer, a second layer over the first layer, a third layer over the second layer, a fourth layer over the third layer, and a fifth layer of the fourth layer, in which the first layer, the third layer, and the fifth layer of the first gate spacer are made of a same material. The second gate spacer includes a first layer, a second layer over the first layer, and a third layer over the second layer, in which the first layer and the third layer of the second gate spacer are made of a same material, and in which a lateral width of the first gate spacer is greater than a lateral width of the second gate spacer.