A method for forming a gate-all-around structure is provided. The method includes forming a plurality of a first type of semiconductor layers and a plurality of a second type of semiconductor layers alternately stacked over a fin. The first type of semiconductor layers includes a first semiconductor layer and a second semiconductor layer, and the first semiconductor layer has a thickness greater than that of the second semiconductor layer. The method also includes removing the second type of semiconductor layers. In addition, the method includes forming a gate to wrap around the first type of semiconductor layers.