Dithienylethene derivatives, which undergo thermally irreversible photochromic reactions, were applied to sensitive color dosimeters. Upon gamma-irradiation, polystyrene films containing diarylethene derivatives, such as 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (1) or 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (2), and fluorescent metal complexes turned blue or red. Even if the absorbed dose was as small as 10 Gy, a clear color change was observed.