Semiconductor structures and method for forming the same are provided. The semiconductor structure includes a substrate and a first channel layer and a second channel layer. The semiconductor structure further includes an isolation structure over the substrate and a first gate structure over the first channel layer and the isolation structure. The semiconductor structure further includes a second gate structure over the second channel layer and the isolation structure and an isolation feature laterally sandwiched between the first gate structure and the second gate structure and extending over the isolation structure. In addition, the isolation feature has a top width and a bottom width that is greater than the top width, and an interface between the isolation feature and the first gate structure includes a curved profile.