Fluorine in mold fluxes causes environmental pollution by dissolution into cooling water or evaporation, which considered seriously hazardous to human health and casting facilities. In order to develop a successful fluorine-free mold flux system, viscosity of borosilicate-based fluxes were investigated in consideration of structural evolution of borates when replacing CaF2 with B2O3 and Na2O. Measured viscosity decreased at the beginning of replacing CaF2 (15.74 to 10.77 wt%) with B2O3 (0 to 4.49 wt%) and then increased drastically further substituting CaF2 (10.77 to 4.71 wt%) with B2O3 (4.49 to 9.02 wt%). An actual determinant of viscosity is not the degree of B2O3 replacement, but rather the portion of tetrahedral borate structures (3-D) which necessarily need support of cations (sodium). Moreover, due to 19F NMR, it is suspected that fluorine tends to interact selectively with sodium, and consequently interferes with formation of tetrahedral borate structures.