The crystallinity of a GaN epitaxial layer on a sapphire substrate after the mechanical ding process was estimated by transmission electron microscopy (TEM) and Raman spectroscopic analysis. TEM observation results showed that, the screw dislocations as a threading dislocation were induced by the mechanical dicing process in the limited area up to approximately 1.2 μm from the dicing-line. On the other hand, the crystal strains were up to approximately 7 μm from the dicing-line edge measured by the Raman spectroscopic analysis. The distance difference between the area of the screw dislocations and of the residual strain is caused by the stress relaxation.