The basic compositions of the samples were selected inside and outside of Na2O-B2O3-SiO2 glass phase separation region, respectively. Nanocrystalline TiO2 was crystallized by melt-phase separation process. The results showed that the phase separation of Na2O-B2O3-SiO2-TiO2 glass system was advantaged to TiO2 nucleation and growth, and then the formation of the nanocrystalline. The main mechanism of TiO2 crystallization is that the rich alkali-boron phase has the lower viscidity and higher particles moving ability than that of parent phase glass. Therefore, the movement rate of TiO2 particle can be promoted and TiO2 crystallization can be accelerated. Moreover, the formation of non-homogeneous nucleus situation and the centralization function of TiO2 have less contribution to TiO2 crystallization.