A fabrication method of magnetic device is provided. A magnetic material is provided. A portion of the magnetic material is selectively irradiated by an energy beam, and reactive gas is introduced simultaneously. The magnetic material being irradiated is melted and solidified to form a solidified layer. An outer layer of the solidified layer reacts with the reactive gas to form a barrier layer, so as to form a magnetic unit including the solidified layer and the barrier layer. It is determined whether the manufacturing process of the same layer is finished, if not, the energy beam is moved to the other portion of the magnetic material. The above step is repeated to overlap multiple magnetic units to form a magnetic layer. If yes, the flow returns to the 1st step to provide another magnetic material to the magnetic layer. The above steps are repeated to form a 3D magnetic device.