Structures and formation methods of a semiconductor device structure are provided. The method includes providing a substrate having a first fin, and the first fin has a channel region and a source/drain region. The method includes forming a stack structure over the first fin, and the stack structure includes a first semiconductor layer and a second semiconductor layer vertically stacked over the fin. The method also includes removing a portion of the second semiconductor layer in the channel region, and a portion of the first semiconductor layer is remaining in the channel region. The method further includes forming a cladding layer over the remaining first semiconductor material layer in the channel region to form a nanostructure, wherein the nanostructure has a dumbbell shape. The method includes forming a gate structure surrounding the nanostructure.