We have studied the effects of total gas velocity during the growth of undoped GaN epilayers on (0 0 0 1) sapphire substrate at a low pressure of 3007 Torr via a two-step growth condition in a horizontal MOCVD reactor. We changed the flow rates of H-2 push gas into the main run line to change total gas velocity on the substrate during the GaN growth. The photoluminescence spectra of undoped GaN epilayers were strongly affected by the changes of the total gas velocity. In a total gas velocity range of 0.21-0.46 m/s under the two-step condition in our study, all samples showed the absence of yellow luminescence band, which was not affected by the changes of the growth temperature from 1080degreesC to 1020degreesC by a 30degreesC decrease. Instead, our samples showed the donor-acceptor pair (DAP) recombination transitions at the extreme conditions in our range of the flow rates of push gas, i.e. total gas velocity. The band-edge emission of I-2, showed a blue-shift with increasing the total gas velocity in a lower velocity regime in this study. The presence of DAP transitions and the peak position of band-edge emission would be a useful indication to obtain a process window, and to optimize the process variables during the growth of an undoped GaN epilayer. (C) 2004 Elsevier B.V. All rights reserved.