Abstract: The presence of significant amounts of molybdenum in high level waste(HLW) streams can lead to the formation of a molybdate salt phase(“yellow phase”) during vitrification. These salts are undesirable from both melter operations and product quality perspectives, such that yellow phase formation can become the primary factor that limits waste loading in the HLW glass product. Improvement of Mo content and elimination of yellow phase in glass by changing glass formulation or developing new formulation are feasible. In this paper, the research progress of the chemical behavior of Mo during vitrification is reviewed.