The invention provides a method for manufacturing a gallium nitride-based III-V group compound semiconductor device, which comprises the following steps: forming a semiconductor stacked structure over a substrate, wherein the semiconductor stacked structure comprises an n-type semiconductor layer, an active layer, and a p-type semiconductor layer; etching the semiconductor stacked structure to expose a part of the n-type semiconductor layer; forming a first electrode on the n-type semiconductor layer, wherein the first electrode comprises an ohmic contact layer, a barrier layer, and a pad layer; performing an annealing process to lower the contact resistance between the first electrode and the n-type semiconductor layer and activate the p-type semiconductor layer at the same time; and forming a second electrode on the p-type semiconductor layer.