Structural properties of epitaxial ZnO/epi-GaN/Al2O3 heterostructures have been studied by transmission electron microscopy and X-ray diffractometry for the first time. The majority of threading dislocations running along the growth direction of ZnO films are screw-type dislocations. At the interface region! a high density of dislocations located mainly on the basal plane was observed. These dislocations show intense dislocation reactions resulting in a rapid reduction of the dislocation density towards the upper region of the film. From X-ray rocking curve measurements, the full-width at half-maximum (FWHM) value of the (1 0 1) phi-scan is larger than that of the (0 0 2) omega-scan and increase with decreasing the ZnO film thickness. The increment of the width of the (1 0 1) phi-scan with decreasing the film thickness from 500 to 150 nm is much larger (0.13 degrees) than that of the (0 0 2) omega-scan (0.04 degrees). Different broadening in (1 0 1) phi-scan and (0 0 2) omega-scan rocking curves are explained in terms of the defect configuration as investigated by transmission electron microscopy(TEM).