Provided is a semiconductor device including a semiconductor substrate, a plurality of semiconductor nanosheets, a plurality of source/drain (S/D) features and a gate stack. The semiconductor substrate includes a first fin and a second fin. The first fin has a first width less than a second width of the second fin, and a top surface of the first fin is lower than a top surface of the second fin. The plurality of semiconductor nanosheets are disposed on the first fin and the second fin. The plurality of source/drain (S/D) features are located on the first fin and the second fin and abutting the plurality of semiconductor nanosheets. The gate stack wraps each of the plurality of semiconductor nanosheets.