An initial substrate structure for forming a nitride semiconductor substrate is provided. The initial substrate structure includes a substrate, a patterned epitaxial layer, and a mask layer. The patterned epitaxial layer is located on the substrate and is formed by a plurality of pillars. The mask layer is located over the substrate and covers a part of the patterned epitaxial layer. The mask layer includes a plurality of sticks and there is a space between the sticks. The space exposes a portion of an upper surface of the patterned epitaxial layer.