Effect of P 2 O 5 -SiO 2 substitution on spontaneous crystallization of SiO 2 -Al 2 O 3 -P 2 O 5 -Na 2 O-MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P 2 O 5 leads to amorphous phase separation (APS), i.e., phosphate-and silicate-rich phases. It is due to the tendency of Mg 2+ to form [MgO 4 ] linking with [SiO 4 ]. Molar substitution of P 2 O 5 for SiO 2 enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na 2 MgSiO 4 is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals.