A semiconductor structure is provided. The semiconductor structure includes a substrate, a plurality of first gate structures, a plurality of second gate structures, a first strained region, and a second strained region. The substrate has a first region and a second region. The first gate structures are disposed in the first region on the substrate. The second gate structures are disposed in the second region on the substrate. The first strained region is formed in the substrate and has a first distance from an adjacent first gate structure. The second strained region is formed in the substrate and has a second distance from an adjacent second gate structure, wherein the second distance is greater than the first distance.