We have studied the effects of thermal convection during the growth of GaN epitaxial layers by horizontal MOCVD reactor. The epilayers were grown with the variation of reaction pressures in order to change the total gas velocity in the horizontal MOCVD reactor. The van der Pauw technique, SEM, double crystal X-ray diffractometry and photoluminescence were used in the characterization of their crystallographic, electrical and optical properties for investigating thermal convection effect of NH3. The GaN epilayer which was grown under thermal convection of NH3 related to low gas velocity shows rough surface having large islands. However, the quasi-mirror-like surface without any defect such as hillocks and islands can be seen in the sample which was grown free from thermal convection of NH3 due to suitable gas velocity. The growth rate is also changed, which results from different thermal convections of NH3 related to various gas velocities in the reactor during their growth. Judging from various data, it is possible to conclude that thermal convection of NH3 is taking place when the epilayer is grown in the reactor at a gas velocity below 0.32 m/ s. And, the generated thermal convection of NH3 Seriously affects the quality of GaN epilayers such as crystallinity, electrical characteristics and optoelectronic properties. (C) 2002 Elsevier Science B.V. All rights reserved.