GaN epilayers nitridated initially for different times have been investigated by light scattering tomography (LST) and Raman scattering. In the LST images of the plan-view epilayers, the light scattering defects mainly distribute in <112I0> directions. The density of the defects is lower in epilayer nitridated initially for a longer time. The defects are considered to be the straight threading edge dislocations on {1100} planes. The Raman shift of E2 mode is larger in the sample initially nitridated for a longer time. Our results show that the misfit between the GaN epilayer and the Al2O3 substrate is more unfavorably accommodated by the threading edge dislocations in the epilayers initially nitridated for a longer time.