In this paper, defect induced by heated treatment in silica is studied. The formation process of defect is analyzed by the thermodynamic model. The results show that defect can be produced by thermal treatment process. The concentration of defect can be affected by many factors. The defect formation induced by heated treatment is related to the initial precursors. To pure silica glass, the precursors mainly come from the strained bonds of network. It can produce centers and nonbridging oxygen hole centers (NBOHCs). These defects are also related to the temperature of heated treatment and the conversion frequency between precursors and defects. The changes of defect concentration with temperature and conversion frequency are also discussed in detail.