A method includes forming a stacked structure of a plurality of first semiconductor layers and a plurality of second semiconductor layers alternately stacked in a first direction over a substrate, the first semiconductor layers being thicker than the second semiconductor layers. The method also includes patterning the stacked structure into a first fin structure and a second fin structure extending along a second direction substantially perpendicular to the first direction. The method further includes removing the first semiconductor layers of the first fin structure to form a plurality of nanowires. Each of the nanowires has a first height, there is a distance between two adjacent nanowires along the vertical direction, and the distance is greater than the first height. The method includes forming a first gate structure between the second semiconductor layers of the first fin structure.