An original high temperature annealing (HTA) technology has been established to fabricate a porous substrate with a layer of inverted pyramid structures. The details about the formation of the porous substrate and the related mechanism were discussed. It was proved that the porous structures were formed through an etching-like mechanism assisted by SiO₂ patterned masks. High-quality GaN crystals have been prepared using the as-prepared porous substrate. The growth experiments demonstrated that such porous substrates were mechanically fragile. The porous structures are suitable to be used as a release layer for the growth of GaN crystals with low stress and defect density.