The effects of LiF and CaF
2 in the vitreous bond have been studied by the measurement of bonding strength. The results show that, the trend of CaF
2 crystallization from Al-Si-O system vitreous bond is more obvious than that of LiF. The melting-point of the bonds is lower and the bonding force is stronger when using LiF than CaF
2 as flux. The mole ratio (Al
2O
3+B
2O
3)/(R
2O+RO+0.5LiF) can be used as a criterion to confirm the self-strength of the vitreous bonds.