Effects of oxide additives such as MoO3, MnO2 ZrO2 and Fe2O3 on the phase separation and the chemical durability of sodium borosilicate glasses which are the host of waste glasses have been investigated as the basic study on the nuclear-waste immobilization through vitrification. MoO3 and MnO2 were found to be phase separation promotors which increased the temperature as well as catalyzed nucleation and growth for the phase separation of the 10Na2-O-3OB2O3-6OSiO2 (wt%) parent glass within the immiscibility region. The glasses had the interconnected phase-separated structure as the amount of addition increased. On the other hand, ZrO2 and Fe2O3 were inhibitors which showed the reverse effects to the above promotors. It was also found that addition of MoO3 could cause the phase separaton of the 20Na2O-10B2O3-70SiO2(wt%) glass even within the miscibility region. Addition of ZrO2 and Fe2O3 increased the chemical durability of the parent glass within the immiscibility region. Within the miscibility region, however, the addition of 1.96 wt % of MoO3 increased the chemical durability considerably, while MnO2 had little effects.