An III-nitride epitaxial structure and a method for manufacturing the same are disclosed. The III-nitride epitaxial structure includes a gallium nitride layer, an indium gallium nitride layer, and an indium nitride layer. The gallium nitride layer includes an M-plane gallium nitride surrounding a c-plane gallium nitride thereof. The indium gallium nitride layer is arranged on the gallium nitride layer. The indium gallium nitride layer includes an M-plane indium gallium nitride surrounding a c-plane indium gallium nitride thereof. The indium nitride layer is arranged on the indium gallium nitride layer. The indium nitride layer includes an M-plane indium nitride surrounding a c-plane indium nitride thereof. The c-plane gallium nitride, the c-plane indium gallium nitride, and the c-plane indium nitride are stacked each other to form a neck portion that is connected to a thin c-plane indium nitride disk which is spaced from the M-plane indium nitride by a gap.