We investigated the influence of low light intensity on the pigmentation of petals in potted lisianthus [Eustoma grandiflorum (Raf.) Shinn.]. When whole plants were placed under 1,000 lx, the petals at anthesis had lighter petal than control flowers exposed to 10,000 15,000 lx. Shading flower buds during the opening process had no effect on petal color intensity. However, shading the stem and leaf parts significantly reduced the intensity of petal color. When flower buds were detached with peduncles and placed in sucrose solutions, their petal color became more intense as the sucrose concentration was increased. There was a positive correlation between anthocyanin and soluble sugar content in the petals, regardless of light conditions. In detached flowers, the transcript levels of chalcone synthase, chalcone isomerase, and dihydroflavonol 4 reductase were enhanced as the sucrose concentration increased. These results suggest that low light intensity affects petal pigmentation through reduced photosynthesis in the leaves or stems, which, in turn, reduces the soluble sugar content of petals and leads to a repression of the genes that encode enzymes of anthocyanin biosynthetic pathway.