The detailed defects and interface in the ZnO films on (1 1 (2) over bar 0) a-plane of sapphire have been characterized using transmission electron microscopy. The single crystal ZnO films are grown by electron cyclotron resonance-assisted molecular beam epitaxy. The orientation relationship between ZnO films and sapphire is (0 0 0 1)(ZnO)parallel to (1 1 (2) over bar 0)sapphire and [2 (1) over bar (1) over bar 0]ZnO parallel to [0 0 0 1](sapphire). A majority of the threading dislocations was found to be screw or mixed. When the interfaces are observed in [0 0 0 1]sapphire direction, the interfaces appear structurally semicoherent with a comparative regular array of misfit dislocations at an interface accommodating a mismatch of about 2.45%. Good matches between simulated and experimental images of the ZnO/a-plane sapphire were obtained. The structural model for the atomic arrangements of the interface was proposed.