Undoped GaN layers were grown on SiN-treated (0 0 0 1) sapphire substrates by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). The thermal decomposition of GaN layers was studied by in situ laser reflectometry and scanning electron microscopy (SEM) analysis. The thermal annealing process was performed in the MOCVD vertical reactor in the temperature range 900-1200 • C in N 2 atmosphere. It was observed that annealing at temperature above 1100 • C causes the decomposition of GaN layers. An activation energy of 3.2 eV was deduced for the GaN thermal decomposition in N 2 atmosphere. The GaN decomposed surface films were analyzed by SEM. These observations were correlated with the in situ laser reflectometry measurements.