Abstract: Methyl methacrylatc-styrene copolymers with high conversion initiated by the radical were cross-fractionated in two solvent-nonsolvent, acetone-acelonitrile and n-butyl chloridc-cyclohcxane, 35 fractions were obtained and their chemical compositions and molecular weights were determined. The results show that the application range of Stockmayer equation should include copolymer with high conversion, if a correction was made to offset the deviation due to the changs of reactivity ratios during the ccpolymerization, a chemical composition distribution curve which is basically in accord with the experimental datum was obtained.