A semiconductor device and a fabrication method thereof are provided. The semiconductor device includes a first nitride semiconductor layer, a second nitride semiconductor layer, a gate structure, and a field plate. The second nitride semiconductor layer is formed on a first surface of the first nitride semiconductor layer. The gate structure is disposed on the second nitride semiconductor layer. The field plate includes a first portion and a second portion connected to the first portion. The first portion has a first surface substantially in parallel to the first surface of the first nitride semiconductor layer, and a second surface adjacent to the first surface of the first portion. The first surface of the first portion of the field plate and the second surface of the first portion of the field plate define a first angle of about 90°.