In this paper, wing tilt was detected by high resolution X-ray rocking curve for uncoalesced GaN epilayer grown on maskless periodically grooved sapphire fabricated by wet chemical etching. Stress distribution was characterized by the frequency shift in micro-Raman spectroscopy measurement. It shows inhomogeneous stress distribution between the mesa and wing region, which is in agreement with the finite-element analysis simulations. Meanwhile, the wing tilt calculated by finite-element analysis agrees well with the rocking curve result. The results show that the different relaxation of stress causes inhomogeneous displacements between the mesa and wing region is the main reason of wing tilt formation. (C) 2006 Elsevier Ltd. All rights reserved.