Growth and structural properties of ZnO films on (10-10) m-plane sapphire substrates by plasma-assisted molecular beam epitaxy were studied. The ZnO films were grown at a wide range of growth temperature of 100-800 degrees C. From the x-ray diffraction theta-2 theta measurements, overwhelming ZnO (10-10) reflection was confirmed in addition to the very weak (0002) and (10-13) reflections. The sample grown at 500 degrees C showed the highest purity with the very weak (10-13) and the negligible (0002) reflections. The full widths at half maximum (FWHMs) for the (10-10) x-ray rocking curves (XRCs) with phi=0 degrees is much higher than those for with phi=90 degrees, which indicates that the films have an anisotropic structural property. The FWHMs for the off-axis (10-11) XRCs were larger than by about 0.1 degrees than those for the (10-10) XRCs with phi=90 degrees. Two distinct features of the striated surface morphology with the grains along the ZnO < 0001 > direction and the inhomogeneity with the appearance of islands on the surface were observed. Detailed investigation on the microstructure by transmission electron microscopy revealed that ZnO (10-13) inclusion was present above the ZnO/Al2O3 interface not at the interface. The ZnO (10-13) inclusion did reach to the top surface, which is believed to the cause for the extra spots in the reflection high energy electron diffraction patterns and the inhomogeneous islands in the atomic force microscope images.