GaN epilayers grown on pre-nitridated (0 0 0 1) sapphire substrates by metallorganic vapor-phase epitaxy were investigated by a high-resolution transmission electron microscope. The lattice images of dislocations with an edge component were classified into two types. One was a regular arrangement of white dots at the core and the other was an irregular distortion of white dots and line segments around the core. The images showed anomalous contrasts with either intense or dim white dots and line segments, which suggested that precipitation had occurred at the cores. The lattice images usually appeared as former type in areas with larger local dislocation intervals and as the latter type in areas with smaller local intervals. This suggested that the nearby edge dislocations change the strain balance at the dislocation and bring about dislocation to climb and/or glide.