We have investigated the time decay of the photoluminescence (PL) at room temperature of high-quality (HQ) and low-quality (LQ) GaN epitaxial layers grown on sapphire by metalorganic chemical vapor deposition. For undoped and Si-doped HQ GaN, the full-width at half-maximum of the (1 0 2) X-ray diffraction curve is 562 and 427 arcsec and the dislocation density is 4 x 10(8) and 2 x 10(8) cm (- 2), respectively. It is found that the PL of HQ GaN has a higher intensity and decays slower than that of LQ GaN. The PL decay time is found to be much longer in HQ GaN. The dual-exponential decay times are 50 and 250 ps for undoped HQ GaN, and 150 and 740 ps for Si-doped HQ GaN. To our knowledge, the decay times of 150 and 740 ps are the longest ever reported in GaN thin films at room temperature. Furthermore, the characteristics of PL decay with different excitation intensities and laser beam focusing conditions are also reported.