We have investigated both elastically and plastically deformed GaN films on lithium gallate, LiGaO(2), by molecular beam epitaxy. The in-plane lattice parameters were determined from high resolution X-ray diffraction and indicated two different groups of in-plane lattice parameters, influenced by the a- and b-axis of LiGaO(2). The measured in-plane lattice parameters indicate that there exist both compressive and tensile strains of in-plane GaN along the a- and b-axis of LiGaO(2), respectively. This anisotropic strain in GaN films forms a slight distortion of the basal-plane hexagonal structure of GaN films, leading to a different critical thickness of 4.0 +/- 0.17 and 7.8 +/- 0.7 nm along the a- and b-axis of LiGaO(2), respectively. (C) 2009 Elsevier B.V. All rights reserved.