The dislocations in electron-irradiated c-plane n-GaN epitaxial layers grown on c-plane sapphire substrates by MOCVD were revealed by several different wet chemical etching methods. And the defect-selective etching method combined with SEM was carried out to study the mechanism of dislocations generation of GaN. SEM images of GaN epilayers with several individual methods are in good agreement with each other. Among all the defects, threading dislocations (TDs) dominated in the GaN epilayers and these defects could be divided into three types. In addition, the EPDs after annealing at various temperatures were studied. The experimental results showed that suitable thermal annealing can eliminate some dislocations.