A photochromic behavior was found for trans-4-diethylamino-4'-dimethylamino-2-hydroxychalcone in toluene, and colored species undergoing dark reaction to regenerate trans-isomer may be ascribed to cis-isomer with the lifetime of 77 min at room temperature. It is demonstrated that a trans-->cis isomerization in the lowest triplet state is significantly responsible for the primary process of the coloration.