Structures and formation methods of a semiconductor device structure are provided. The semiconductor device structure includes a substrate, a first source portion and a first drain portion over the substrate, and a first semiconductor nanowire over the substrate and between the first source portion and the first drain portion. The first semiconductor nanowire includes a first portion over the substrate and a second portion over the first portion, and the first portion has a first width, and the second portion has a second width, and the second width is less than the first width. The semiconductor device structure also includes a first gate structure over the second portion of the first semiconductor nanowire.