Reversible thermochromic properties were observed in Bi2O3-Li2O system glasses. The temperature coefficient of optical absorption edge increases with increasing Bi2O3 content, implying that Bi2O3 is the main reason of thermochromism. The thermochromic mechanism is explained in terms of the temperature dependence of the optical energy gap Eg, caused mainly by the electron-phonon interaction. The low phonon energy and high electron density of Bi—O bond are important factors contributing to the electron-phonon interaction in these glasses.