The article reports fabrication, characterization and testing of the thermal stability of ZnO-based Schottky ultraviolet photodetectors. The ZnO thin film was grown on p-type Si ‹100 › substrate by sol-gel technique. The surface morphological and the structural properties of the thin film were studied by atomic force microscope (AFM) and scanning electron microscope (SEM). For the investigation of the surface chemical bonding, X-ray photoelectron spectroscopy (XPS) measurements were also performed. The I-V characteristics of the Schottky barrier photodetector were studied and the parameters such as ideality-factor, leakage-current, and barrier-height were extracted from the measured data at room temperature. With applied bias voltages in the range from -3V to 3V the contrast-ratio, responsivity, detectivity and quantum-efficiency of the photodetector were measured for an incident optical power of 0.1mW at 365nm wavelength. The electrical and optical study revealed that the performance of the device improves with increasing post metal deposition annealing temperature up to 100˚C. The device exhibited excellent thermally stability in the annealing temperature range of 100˚C to 200˚C. For annealing temperature beyond 200˚C the performance of the device degrades drastically. It was also found that under 200˚C there is a harmonious relation between the optical and electrical characteristics of the device. Above this annealing temperature there is no correlation between the variations of optical and electrical characteristics with increasing annealing temperature. The variation of the electrical and photoresponse properties of Schottky photodetector subjected to different post fabrication annealing can be attributed to combined effects of interfacial reaction and phase transition during the annealing process.