Oxide glasses having lower glass transition temperature are widely used in electric devices. The oxide glasses usually contain lead oxide. Recently, lead oxide glasses have been replaced with lead-free oxide glasses because of the environmental pollution. We investigated the thermal properties and the glass formation region in the Bi2O3-B2O3-ZnO ternary system. The results showed that the ratio of T-g/T-l of the bismate glasses is around 0.55, which is lower than the value of ratio 0.67, the two-third rule (T-g/T-l=2/3) in other glasses, and the glass formation region is in the range of 20-60 mol% contents of B2O3.