As an optical film, chromium fluoride is an important material because it has good transparency in short wavelength region. However, it was found that chromium fluoride film degrades at humid condition. Therefore, we investigated the degradation phenomena of chromium fluoride films under 100% humid condition. The degraded samples were analyzed by several thin film analyzing techniques. With these results, we proposed a possible degradation mechanism consisting of multi-step reactions. First, H2O molecules are permitted to the porous structure in the films to percolate through numerous microchannels and microvoids. Then, the water molecules react with the chromium fluoride film to make hydroxide bonding producing HF gas. HF gas evolves and gathers at some spots, and dome structure was developed. Chromium oxide precipitates around HF gas gathered dome. After time passes, HF gas evolution occurs followed by explosion of the dome structure, and crater-like feature remains as a result around the dome. It was found that the chromium oxide ring is left around the feature. In the whole process, cracking also occurred due to the molecular volume difference between CrFx and degraded part.