Study on initial growth of GaN revealed evolutional improvement of grain structure in the initial stage of ammonothermal growth. GaN crystals grown on N-polar surface of GaN platelets for various growth durations were characterized with the X-ray diffraction and Nomarski microscopy. The full width half maximum (FWHM) of X-ray rocking curve from 002 reflection became larger than that of a seed crystal at approximately 100 mu m, followed by improvement in thickness larger than approximately 200 mu m. On the other hand, FWHM of X-ray rocking curve from 201 reflection did not change. The Nomarski microscope showed larger hillocks for thicker layer indicating coalescence of grains. These results imply that GaN crystal quality in the initial stage was improved through reduction of c-axis tilting associated with coalescence of grains in the grown layer. (C) 2011 Elsevier B.V. All rights reserved.