Disclosed are a GaN (gallium nitride) compound power semiconductor device and a manufacturing method thereof. The gallium nitride compound power semiconductor device includes: a gallium nitride compound element formed by being grown on a wafer; a contact pad including a source, a drain, and a gate connecting with the gallium nitride compound element; a module substrate to which the nitride gallium compound element is flip-chip bonded; a bonding pad formed on the module substrate; and a bump formed on the bonding pad of the module substrate so that the contact pad and the bonding pad are flip-chip bonded. By this configuration, it is possible to reduce the process costs by forming the bump on the substrate based on the wafer level, rapidly emit the heat generated from an AlGaN HEMT device by forming the sub source contact pad and the sub drain contact pad of the substrate in the active region, and efficiently emit the heat generated from the AlGaN HEMT device by forming a via hole on the substrate and filling the via hole with the conductive metal.