Surface　pits　in　a-plane　GaN　have　long　been　considered　as　a　result　of　island　coalescence　in　literature,　while　pits　in　c-plane　GaN　are　the　surface　termination　of　screw　dislocations.　It　is　found　in　the　paper　that　small　surface　pits　on　a-plane　GaN　also　terminate　perfect　screw　dislocations　and　partial　dislocations　(PD),　which　is　due　to　the　intrinsic　nature　of　the　spiral　growth　of　screw　dislocations.　This　resulthas　demonstrated　that　a-plane　GaN　also　follows　the　prediction　based　on　Frank＇s　classical　theory　of　dislocated　crystals.　Moreover,　although　caused　by　dislocation,　the　relative　size　of　the　small　pits　will　be　affected　by　the　kinetic　process　of　the　atomic　reactions　on　the　surface　through　V/III　ratio　variation.　Meanwhile,　large　hexagonal　surface　pits　are　observed　on　c-plane　GaN　grown　under　low　V/III　ratio,　which　is　formed　under　the　same　mechanism　as　the　large　triangular　or　pentagonal　pits　on　a-plane　GaN　surface.　During　the　island　coalescing,　the　lateral　growth　of　the　inclined　facets　are　in　competition　with　the　vertical　growth　in　the　epitaxial　orientation,　so　incomplete　island　coalescence　will　produce　large　surface　pits　for　both　a-and　c-plane　GaN.　Under　this　mechanism,　a　triangular　pit　of　a-plane　GaN　could　be　transformed　into　a　pentagonal　pit　upon　increasing　temperature.