As potential substitutes for fluorides, the influences of basicity and B2O3 content on viscosity, melting and crystallization behaviors of low fluorine mold fluxes for casting medium carbon steels were investigated by using Brookfield viscometer and Single Hot Thermocouple Technique in this study. Results suggested that, the break temperature, crystallization temperature and critical cooling rate of low fluorine mold fluxes were increased with the increase of basicity; while they were decreased with the further addition of B2O3. Meantime, the viscosity and melting temperature range were first attenuated, and then increased greatly with the increase of basicity; however, they would tend to be reduced with the addition of B2O3 content. Also, it was found that the viscosity of mold flux is not only decided by the degree of polymerization of silicate structure, but also greatly affected by its crystallization behavior.