Dimethyl methylphosphonate (DMMP) was used as a flame retardant additive to 1M LiPF6/EC+DEC system. The flammability, electrochemical stability and cycling performance of electrolyte containing DMMP were studied. The addition of DMMP to electrolytes provides a significant suppression in the flammability of the electrolyte concluded from the measurements of self-extinguish time and limited oxygen index. The totally nonflammable electrolytes can be achieved with only 10wt.% DMMP addition—the highly efficient retardant additive. The addition of DMMP causes little damage on the cell electrochemical performance. DMMP is a promising flame retardant additive to improve the safety of lithium-ion batteries.