A semiconductor device includes a semiconductor substrate, an isolation structure, a cladding layer, and a gate structure. The semiconductor substrate includes fin shaped structures. The isolation structure is disposed between the fin shaped structures. Each of the fin shaped structures includes a first portion disposed above a top surface of the isolation structure and a second portion disposed on the first portion. A width of the second portion is smaller than a width of the first portion. The cladding layer is disposed on the first portion and the second portion of each of the fin shaped structures. The cladding layer includes a curved surface. The gate structure is disposed straddling the fin shaped structures.