In thermal Molten Salt Reactors, the nuclear graphite core is in direct contact with the molten salt coolant. Due to the porous nature of nuclear graphite, the molten salt may infiltrate the nuclear graphite, which may affect the mechanical strength and irradiation behavior of the nuclear graphite. In order to evaluate the infiltration behavior of molten salt in nuclear graphite, both FLiNaK and FLiBe salts were used to infiltrate two typical nuclear graphite grades: IG110 and NBG18. The pressure dependence of the infiltration weight gain ratio was measured. The influence of molten salt infiltration on the thermal properties of these two graphite grades, such as their thermal expansion behavior and thermal conductivity, was also measured. The mechanical strength of the FLiNaK-infiltrated graphite was measured at room temperature and elevated temperature, and showed that the mechanical strength of the nuclear graphite was enhanced at room temperature and weakened at elevated temperature by molten salt infiltration. Finally, the thermal expansion coefficient and the fracture surface analysis measured after FLiNaK infiltration indicated that the stress induced by molten salt infiltration could be one of the reasons for the graphite property changes.