A semiconductor device structure includes a fin structure, a semiconductive capping layer, an oxide layer, and a gate structure. The fin structure protrudes above a substrate. The semiconductive capping layer wraps around three sides of a channel region of the fin structure. The oxide layer wraps around three sides of the semiconductive capping layer. A thickness of a top portion of the semiconductive capping layer is less than a thickness of a top portion of the oxide layer. The gate structure wraps around three sides of the oxide layer.