A semiconductor device includes an interfacial layer on a substrate and agate structure on the interfacial layer. Preferably, the gate structure includes a patterned high-k dielectric layer, the patterned high-k dielectric layer comprises a metal oxide layer, and a horizontal direction width of the patterned high-k dielectric layer and a horizontal direction width of the interfacial layer are different. The semiconductor device also includes a first spacer adjacent to the gate structure and on part of the interfacial layer and contacting a top surface of the interfacial layer and a second spacer on the sidewalls of the first spacer and the interfacial layer. Preferably, a planar bottom surface of the second spacer is lower than a planar bottom surface of the first spacer and extending along a same direction as the planar bottom surface of the first spacer.