A semiconductor device and a manufacturing method thereof are provided. The semiconductor device includes an oxide semiconductor protrusion, a source, a drain, an oxide semiconductor layer, a first O-barrier layer, a gate electrode, a second O-barrier layer, and an H-barrier layer. The oxide semiconductor protrusion is disposed on an oxide substrate. The source and the drain are respectively disposed on opposite ends of the oxide semiconductor protrusion. The oxide semiconductor layer is disposed on the oxide substrate and covers the oxide semiconductor protrusion, the source, and the drain. The first O-barrier layer is disposed on the oxide semiconductor layer. The gate electrode is disposed on the first O-barrier layer and across the oxide semiconductor protrusion. The second O-barrier layer is disposed on the gate electrode. The H-barrier layer is disposed on the oxide substrate and covers the second O-barrier layer.