Abstract: For understanding the natural configuration and the function of phycoerythrobilin a model compound of biladiene a, b, dihydromesobiliviolin IX_a dimethyl ester, was irradiated with visible light or UV radiation. The absorption change from 565 nm to 610 nm and the ¹H NMR studies on the chemical shifts of C-5 H and C-10 H indicated that a configurational change from Z type to E type at the double bond of C-10 bridge may take place. The molecule has a more closed spiral configuration before irradiation and a more extended configuration after irradiation.