Mercuric Iodide (HgI2) is a promising semiconductor material for nuclear radiation detectors working at room temperature, especially for x-ray and γ-ray detectors. The influences of different growth temperatures on qualities of thin films were studied. The structure and optical properties of thin films were characterized by x-ray diffraction spectroscopy, metallography and UV-VIS spectrophotometer. Our results can be summarized as following: XRD analysis shows crystallinity of HgI2 in thin films depends mainly on the growth temperatures, that is, the XRD reflections become stronger with the decrease of the growth temperature. The optimum growth temperature for preparation of polycrystalline HgI2 thin film utilizing vertical deposition technique of chemistry is about 20°C. The corresponding thin film has a good uniformity with thickness of about 800 nm, perpendicular to the substrate along <001> direction. Based on its optical performance testing, our calculations found that HgI2 thin film grown at 20°C has a wide energy band gap of about 2.26 eV.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.