We report the structures of ZnO/GaN heterointerfaces and properties of ZnO films grown by plasma-assisted molecular beam epitaxy (P-MBE) on (0001) GaN/Al2O3 substrates. At first, different preparations (zinc or oxygen-plasma pre-exposures) of the GaN surface are examined and the resultant heterointerface structures and properties of ZnO films are studied. The oxygen-plasma pre-exposure of the GaN surface results in the formation of an interface layer, while the formation of interface layers is prevented by zinc pre-exposure. The interface layer is identified as single-crystalline monoclinic Ga2O3 through extensive transmission electron microscopy (TEM) study. The orientation relationship between monoclinic Ga2O3 of the interface layer and GaN and ZnO epilayers is investigated as (0001) GaN parallel to(001) Ga(2)O(3)parallel to(0001) ZnO and [2-1-10] GaN parallel to[010] Ga(2)O(3)parallel to[2-1-10] ZnO. Based on structural investigations and stability of bondings, we propose interface-bonding sequences at the ZnO/GaN heterointerfaces in zinc or oxygen-plasma pre-exposed samples: "N-Ga-O-Zn" bonding sequence for zinc pre-exposed samples and "N-Ga-(O-terminated Ga2O3 layer)-Zn-O" for oxygen-plasma pre-exposed ones. Lattice misfit of Ga2O3 against underlying GaN (against upper ZnO) is estimated as -4.7% (-6.5%) and 10.7% (8.6%) along the [010] and [100] directions of the Ga2O3 respectively. The structural and optical properties of ZnO films evaluated by TEM, high-resolution x-ray diffraction and photoluminescence spectroscopy consistently reveal better crystal quality of zinc pre-exposed ZnO films than oxygen-plasma pre-exposed ones. Second, the effects of high temperature annealing on ZnO films grown at low temperature on zinc pre-exposed GaN surface are studied. Surface morphology is dramatically changed by high temperature annealing; three-dimensional island morphology on as-grown ZnO layers changes into atomically flat morphology with atomic level steps and terraces through high temperature annealing. Optical properties are also improved by high temperature annealing: (i) The intensity of band edge emissions from the ZnO films increases with their full width at half maximum values being decreased. (ii) Free exciton emission from annealed films is considerably enhanced in intensity compared to as grown films. These results indicate that high temperature annealing greatly improve the crystal quality of ZnO films grown at low temperature.