There are two main principles of UV light emission of ZnO thin films fabricated by different methods:micro-resonant cavity light emission and random light emission. The variation of the intensity of light emission of ZnO thin films grown by the two methods with detection angle was studied at the same condition and the variation tendency of the intensity was also analyzed. It is proved that they have different principles because of the different crystal structures. ZnO thin films grown by pulsed laser deposition have hexangular column honey comb like structure and its column borders play the role of light amplification reflecting mirror and form micro-resonant cavity. So the lateral surface has the strongest light emission. ZnO thin films grown by thermal oxidation have granular nanomicro-crystal structure and photons are scattered through grains. Light amplified emission occurs from the randomly scattered closed paths. So its intensity of light emission varies with the detection angle slowly.