By considering characteristics of cydo—copolymerization，the composition equation at low conversion was derived in the following form：where [M] and [B] represent the concentrations of diene and the vinyl monomer respec-tively，[M]cop and [B]cop the concentrations of diene and vinyl segment in the copolymer，and R1 and R2 represent the relative reactivity ratio of diene and vinyl monomer respec-tively．When [M]=[B]，then R1 becomes the rate ratio of radical M·or m·attacking diene (including monomolecular and bimolecular mechanism) and vinyl monomer；and R2 becomes the rate ratio of radical B attacking vinyl monomer and diene (including two kinds of mechanism)．By ap-piing the above equation，values of R1 and R2 for 12 sets of monomer pairs were obtained and found to be rather the best values with a permissible range of error．