Despite the excellent flame retardancy of halogen-based flame retardants, harmfulness to the human health leads to the developments of various alternative flame retardant system including phosphorus-based flame retardant. Phosphorus-based flame retardant, however, also has some drawbacks such as weight loss due to volatilization which prevent the formation of an effective carbon barrier of solid phase flame retardant mechanism. To prevent this shortcomings, triethyl phosphate was introduced to the emulsion copolymerization of styrene and acrylonitrile by using maleic anhydride as a modifier. The efficiency of this polymeric phosphorous-based flame retardant was verified by applying to polypropylene during extrusion. This work was supported by Korea Institute for Advancement of Technology(KIAT) grant funded by the Korea Government(MOTIE) (P0017002, HRD Program for Industrial Innovation))