The strain-stress analysis of Al xGa 1-xN/GaN (x = 0.3) heterostructures with and without a high-temperature HT-AlN interlayer (IL) grown on sapphire (Al 2O 3) substrates and AlN buffer/Al 2O 3 templates via metal organic chemical vapour deposition (MOCVD) was carried out based on the precise measurement of the lattice parameters by using high-resolution X-ray diffraction (HRXRD). The a- and c-lattice parameters were measured from the peak positions that were obtained by rocking the theta axis at the vicinity of the symmetric and asymmetric plane reflection angles, followed by the in-plane and out-of-plane strains. Then, the biaxial and hydrostatic components were extracted from the total strain values that were obtained and were then discussed in the present study as the effect of the HT-AlN buffer and IL. The AlN buffer layer (BL) affects the strain values of the AlGaN ternary layer (TL). A further effect was realized by inserting an AlN IL between GaN BL and AlGaN TL. However, the experimental results also show that an AlN IL changes the strain behaviour in the a- and c-directions of the AlGaN TL from the tensile to compressive and the compressive to tensile type, respectively. These similar behaviours were observed in hydrostatic strain, biaxial strain and stress. Their reasons are explained with an effective a-lattice parameter, post-growth cooling and lattice and thermal mismatches. © 2010 Blackwell Publishing Ltd.