In the vitrification of high-level radioactive waste, it is important to understand the crystallization behavior because precipitation of molybdenum (Mo) phase reduces the chemical durability of the waste form. In this study, the crystallization of Mo phase under slow cooling conditions was investigated with glasses of SiO 2 B 2 O 3 Al 2 O 3 CaONa 2 OMoO 3 systems and simulated high-level waste (HLW) compositions. When the B 2 O 3 in the glass decreases and the Al 2 O 3 increases, the phase separation of Na 2 MoO 4 and the crystallization of powellite (CaMoO 4 ) were promoted. The waste components included in the HLW glass has the effect of increasing the MoO 3 solubility and suppressing crystallization. Furthermore, it was found that the suppressing effect was further enhanced by addition of rare earth element oxides. In contrast, there was no suppressing effect of addition of V 2 O 5 on the crystallization of powellite. The precipitation of the Na 2 MoO 4 is mainly related to liquidliquid immiscibility, while the crystallization of powellite depends on the multiple factors, an equilibrium MoO 3 solubility at high temperature, nucleation and growth rates under slow-cooling and crystallization of Ca-silicate phase. The compositional effects on the crystallization can be described using the melt viscosity.