Oxyfluoride glass-ceramics have been introduced as one of the best bulk hosts for upconversion process of rare-earth ions. However, less attention to the technological topics in the process of glass crystallization has hindered the development of them in industrial scale applications. In this research, SiO2-PbF2 glasses were investigated to clarify the ambiguous role of rare-earth ions and alumina content in the microstructure and crystallization behavior. Results indicated that Er3+ addition caused a liquid-liquid phase separation via nucleation and growth mechanism, which led to single phase crystallization of -PbF2:Er3+ solid solution. Moreover, Er3+ had a significant effect on the crystallite size, size distribution, and PbF2 crystallization temperature. On the other hand, increasing Al2O3 content enhanced the transparency and thermal stability of glass samples, whereas it reduced the amount of fluorine loss and increased the dissolution of Er3+ in fluoride crystalline structure. These results address some of the most controversial issues about crystallization behavior of rare-earth-doped oxyfluoride glasses.