||Our recently developed MBE-grown ZnSSe-based visible-blind UV detectors can offer numerous applications in industry, scientific research as well as a number of commercial products. However, the noise characteristics and detectivity of these detectors, which reflect their ultimate limit on small-signal detection, has not been studied prior to this study. This work started with the construction of a hardware system for achieving noise characterization with high sensitivity, followed by a thoughtful study of the noise characteristics and detectivity of a group of ZnS0.58Se0.42 detectors. In total 27 detectors were selected from different locations of a 2-inch MBE-grown Schottky barrier structure fabricated on a GaP (100) wafer. The results of low frequency noise measurements reveal that these detectors can be classified into three different groups based on their different noise power spectrum: Group-A consists of 16 devices all show noise level below the background level of the detection system (about 10-26 A2/Hz) even at -12V; Group-B consists of 6 devices with detectable noise at reverse bias at or larger than 6V; Group-C consists of 5 devices with detectable noise even at -3V. We attribute this non-unifomity to the different damage level caused by wire bonding process. One typical detector from each of the three groups was selected for further detailed studies. The responsivity measurement shows that they all have the cut-off wavelength at about 400nm with very good rejection power for longer wavelength. The responsivity in the UV spectral range is around 0.1A/W. The impulse response measurements indicate that spectral range is around 0.1A/W. The impulse response measurements indicate that the detectors have a response time around 1.4ns and this value is basically unchanged from 0 to -12V. The bandwidth of the diodes was found to be about 300MHz by applying Fourier transformation on the measured impulse response spectra. The I-V characteristics of these detectors were measured for extracting various parameters for the determination of the shot noise power. Finally, the total noise power of these detectors were evaluated, and the best values of noise equivalent power and specific detectivity are calculated to be 2.8 x 10-11W and 1.7 x 1011 mHz1/2W-1, respectively. These values are among the best reported so far from both II-VI and GaN-based Schottky UV diodes and also about two orders of magnitude better than the best value provided from commercial products of Si photodiodes. These results indicate that ZnSSe photodetectors are highly promising for visible-blind UV detection applications requiring extremely high detectivity.