An ohmic metal for GaN device comprises a diffusion barrier seed metal layer and a plurality of metal layers. The diffusion barrier seed metal layer is formed on an epitaxial structure layer. The diffusion barrier seed metal layer is made of Pt. The epitaxial structure layer is made of AlGaN or GaN. The plurality of metal layers is formed on the diffusion barrier seed metal layer. The plurality of metal layers comprises a first metal layer and a second metal layer. The first metal layer is formed on the diffusion barrier seed metal layer. The first metal layer is made of Ti. The second metal layer is formed on the first metal layer. The second metal layer is made of Al. By the diffusion barrier seed metal layer, so as to suppress the diffusion of the plurality of metal layers into the epitaxial structure layer.