We investigated third-order optical nonlinearities (chi((3))) by femto-second Z-scan technique for various compositions of bismuth-tungsten-tellurite glasses (xBiO(1.5)-20WO(3)-(80-x)TeO2; x = 0-15) as well as their Raman scattering spectra. It was revealed that a small additive of BiO1.5 (5 mol%) in WO3-TeO2 glass was capable of improving a real part of chi((3)). However, an excess amount of BiO1.5 decreased the nonlinearity, resulting from TeO4-to-TeO3+1 transformation with an increase in BiO1.5. We shall attempt to explain the improvement on a basis of a two-phases model for WO3-TeO2 glasses.