A light-emitting device of Group III nitride-based semiconductor comprises a substrate, a first Group III nitride layer and a second Group III nitride layer. The substrate comprises a first surface and a plurality of convex portions protruding from the first surface. Each convex portion is surrounded by a part of the first surface. The first Group III nitride layer is jointly formed by lateral growth starting at top surfaces of the convex portions. The second Group III nitride layer is formed on the first surface, wherein a thickness of the second Group III nitride layer is less than a height of the convex portion. Moreover, the first Group III nitride layer and the second Group III nitride layer are made of a same material.