Clear results on the origin of etch pits are reported. The surfaces of high-quality GaN epitaxial layers were etched with molten KOH and observed by atomic force microscopy (AFM) and plan-view and cross-sectional transmission electron microscopy (TEM). AFM images for as-grown samples show dark spots indicating mixed dislocations. For etched samples, hexagonal-base etch pits were clearly observed, and the dark spots disappeared. TEM observations show that a mixed dislocation terminates at the bottom of each etch pit. The densities of etch pits and mixed dislocations are almost the same, i.e., around 3 x 10(8) cm(-2). The origin of etch pits is the mixed dislocation, and the combination of KOH etching and AFM is found to be a better approach for two-dimensional evaluation of mixed dislocations.