In this article. We report on the study of mosaic structures of different thick GaN films grown on sapphire (0001) by metalorganic chemical vapor deposition (MOCVD), using high resolution x-ray diffraction. The result from the symmetrical reflections show that the mosaic vertical and lateral correlation lengths that are calculated by two methods increase with film thickness increasing, and the vertical correlation lengths are close to the film thickness, and the same trend in the lateral correlation lengths derived from the reciprocal space maps. By the help of asymmetrical reflections and Williamson-Hall extrapolation method, the tilt and twist mosaic drop with thickness increasing at different rates. All this shows that the increase in thickness lads to the more uniform and neat grain arrangement and the higher-quality epitaxial wafers.