High quality ZnO films were depos ited on p-GaN by pulsed laser deposition method to form a pn heterojunction UV photodetector and they were tested under UV illumination.scan electro microscopy (SEM) and X-ray diffraction (XRD) results show the good crystal quality of ZnO films.The I-V character istic curves of the n-ZnO/p-GaN heterojunction UV photodetector indicate obvious rectifying behaviour both in the dark and under i llumination.The spectral responsivity curves under several reverse voltages show the spectral responsivit y peak is located at 364nm. When the reverse voltage reaches －5V,the photocurrent of the detector saturate s and the peak responsivity reaches 1.19A/W.The detectivity curves under various reverse volta ges show that the detector has a high selectivity for UV light around 364nm,and the optimal detec tivity reaches 8.9×10¹⁰ cm·Hz1/2/W at －2V reverse bias.At －2V bias,the current of the detector under 365nm UV light is about 183times of tha t in the dark.Continuous measurements indicate the reproducibility and stability of the developed heterojunction UV photodetector.