Glasses with composition xBi2O3–24Na2O–(75−x)B2O3–1Er2O3(where x = 0, 5, 10, 15, 25, 33, 40 mol%), were prepared by the melt quenching technique. The effect of Bi2O3content on thermal stability, optical properties and structures of these glasses is systematically investigated by X-ray diffraction, infrared spectroscopy and DTA techniques. The variations in the optical band gap energies, with Bi2O3content have been discussed in terms of changes in the glass structure. Urbach energy increases with increasing Bi2O3content in the present glass system. It is found that the density, molar volume and optical basicity increase with increasing Bi2O3. The glass transition temperature (Tg) of the samples was found to decrease with the Bi2O3content. IR measurements revealed an existence of trigonal BO3pyramid, tetrahedron BO4, pyramidal BiO3and octahedron BiO6structural units in the network of the investigated glass. Furthermore, a decrease in BO4and an increase in BO3take place against the increase of x which means that, Bi2O3plays the role of network modifier in the structural network.