This work examines the anisotropic microstructure and the lattice distortions of nonpolar a -plane $$(11\bar 20)$$ GaN ( a -GaN) films by using the grazing-incidence X-ray diffraction technique. Faulted a -GaN films typically exhibit an in-plane anisotropy of the structural properties along the X-ray in-beam directions. For this reason, the anisotropic peak broadenings of the X-ray rocking curves (XRCs) were observed for various angle (phi) rotations for a -GaN films with and without SiN x interlayers. Analysis revealed the peak widths of the XRCs displayed an isotropic behavior for a nonpolar a -GaN bulk crystal. Thus, the in-plane anisotropy of the XRC peak widths for nonpolar a -GaN films apparently originates from the heteroepitaxial growth of the a -GaN layer on a foreign substrate. The lattice distortion analysis identified the presence of compressive strains in both the two in-plane directions (the c - and the m -axis), as well as a tensile strain along the normal growth direction. In addition, the observed frequency shifts in the Raman E 2 (high) mode for the a -GaN films showed the existence of considerable in-plane compressive strain on both a -GaN films, as confirmed by the lattice distortion analysis performed using the grazing-incidence XRD method.