Viscosity and crystallization are essential properties to characterize the lubrication and heat transfer performances of mold flux. Therefore, in this paper, the viscosity and crystallization behaviors of conventional F-containing commercial mold flux and newly designed F-free mold fluxes were investigated by using rotating cylinder method and Single/Double Hot Thermocouple Technique (SHTT/DHTT). Results shows that the viscosities of designed F-free mold fluxes are close to the F-containing mold flux; and the crystallization temperatures of F-free mold fluxes increases with the increase of basicity and Na₂O/Li₂O content; while it decreases with the increase of cooling rate and the addition of B₂O₃. The final steady state structure of F-free mold fluxes during the DHTT tests shows it is composed of a major crystalline and a thin liquid layer (5.42–7.2%) without glass phase. The results of XRD indicate that the main crystalline phases formed in designed F-free mold fluxes were calcium borosilicate (Ca₁₁Si₄B₂O₂₂) and calcium magnesium silicate (Ca₁₄Mg₂(SiO₄)₈. Through the comprehensive comparison, the designed F-free mold fluxes flux Sample E (Basicity 1.15, Na₂O 7.92, Li₂O 1.97 and B₂O₃ 5.98) has the closest performances to the benchmark conventional mold flux Sample A.