In this study, we investigated the application of a novel in situ X-ray diffraction (XRD) monitoring method to control GaN crystal growth using a low-temperature (LT)-deposited buffer layer. We found that this method is useful in controlling the annealing of the LT-buffer layer, which strongly depends on the crystallinity of GaN. Accordingly, if we employ in situ XRD grown on GaN using LT-buffer layer on sapphire substrate, the optimization of the annealing conditions will become easier because it would be possible to determine by only one growth procedure. Therefore, we expect that this method will serve as a new and helpful optimization tool for crystal growth. (C) 2013 Elsevier B.V. All rights reserved.