The origin of threading dislocations (TDs) in GaN epitaxial layers grown on sapphire (0001) substrate is investigated using moire fringes from plan-view transmission electron microscopy. The studied samples are nucleation layers deposited at 560 degrees C for times ranging from 20 s to 180 s. This initial stage growth gives rise to islands which are randomly rotated and relaxed with misfit dislocations. The islands that start to coalesce from 60 s growth time keep this random orientation and this leads to the bending of 60 degrees misfit dislocations in the interface plane to form a-type TDs inside low angle boundaries. (c) 2005 American Vacuum Society.