The effect of biaxial strain on optical phonons in high-quality GaN epitaxial layers grown on 6H–SiC substrates by metal organic chemical vapor deposition has been studied. The deformation potential constants for theE2(1),A1(TO),E1(TO),andE2(2)optical phonon modes in hexagonal GaN have been obtained. A method for calculating strain in hexagonal GaN layers from Raman data alone is suggested. A comparative analysis of the strain measured by x-ray diffraction and Raman spectroscopy shows that these data agree well. It is found that the biaxial stress of 1 GPa results in a shift of the excitonic photoluminescence lines of 20±3 meV. ©1997 American Institute of Physics.