The phase-transfer radical copolymerization of acrylonitrile and methylacrylate with hexadecyltrimethyl ammonium bromide was investigated in (NH4)2S2O8 aqueous-organic two-phase system at 55℃ and under nitrogen atmosphere. The rate of copolymerization was expressed in terms of quaternary ammonium ion and peroxydisulfate ion in the aqueous phase rather than the fed concentrations of HTAB and (NH4)2S2O8. The observed initial rate of copolymerization was used to analyze the copolymerization mechanism with a cyclic phase-transfer initiation step.