The relationship between the viscosity and structure of B2O3-containing calcium-silicate-based mold fluxes and the effects of fluidizers including CaF2, CaO, and B2O3 on the viscosity and their correlation with the structural aspects were studied using a rheometer with Fourier transformation infrared and Raman spectroscopy. The viscosity decreased with increasing CaF2 addition up to 28 wt% at a fixed CaO/SiO2 ratio of 0.3, which was related to depolymerization. Furthermore, CaF2 addition also affected the apparent activation energy for viscous flow, which decreased with increasing CaF2 content to 105.1 from 151.1 kJ/mol. At higher C/S ratios, the viscosity decreased in the presence of greater Ca2+ and O2- supplied from CaO, which subsequently increased the activation energy to 149.7 from 122.0 kJ/mol. With regard to the B2O3-melt, polymerization of the network structure was observed by comparing the B2O3-free to 4.4 wt% B2O3 content. However, the viscosity was relatively constant with increasing B2O3 addition. However, the viscosity decreased due to greater simplification of the network structure above 4.4 wt% B2O3. The break temperature decreased with greater B2O3 addition as the crystallization was inhibited. Furthermore, the apparent activation energy decreased as depolymerization of the network structures occurred above 4.4 wt% B2O3.