Online low emissivity glass (Low-E glass) is a promising glass product. Determination of the optical constants of the functional layer of online Low-E glass is of practical interest for many applications. In this paper, the optical property mechanism of online Low-E glass was analyzed. The expression for the relationship between transmittance and optical constants was derived on the basis of the Drude theory and the basic methods for thin film optics. Finally, the optical constants of the functional layer of online Low-E glass were obtained by fitting the measured transmission spectrum using the theoretical formula. In addition, the free-carrier concentration of the functional layer was calculated based on Burstein-Moss theory. The calculated results are consistent with the experimental results. (c) 2007 Elsevier B.V. All rights reserved.