We report measurements of stress in GaN epitaxial layers grown on 6H–SiC and a-Al2O3 substrates. Biaxial stresses span 11.0 GPa ~tensile! to 21.2 GPa ~compressive!. Stress determined from curvature measurements, obtained using phase-shift interferometry ~PSI! microscopy, compare well with measurements using accepted techniques of x-ray diffraction ~XRD! and Raman spectroscopy. Correlation between XRD and Raman measurements of the E2 2 phonon gives a Raman-stress factor of 23.460.3 cm21/GPa. We apply PSI microscopy for temperature dependent stress measurements of the GaN films. Variations found in the stress–temperature coefficient correlate well with threading dislocation densities. We develop a phenomenological model which describes the thermal stress of the epitaxial GaN as a superposition of that for ideal GaN and the free volume existing in the layers due to the threading dislocations. The model describes well the observed dependence.