Semiconductor structures and method for forming the same are provided. The semiconductor structure includes a substrate and first nanostructures and second nanostructures formed over the substrate. The semiconductor structure further includes a first source/drain structure formed adjacent to the first nanostructures and a second source/drain structure formed adjacent to the second nanostructures. The semiconductor structure further includes a first contact plug formed over the first source/drain structure and a second contact plug formed over the second source/drain structure. In addition, a bottom portion of the first contact plug is lower than a bottom portion of the first nanostructures, and a bottom portion of the second contact plug is higher than a top portion of the second nanostructures.