Abstract: In this paper, the primary chains are regarded as the basic unit constituting a macromolecule. There upon it, the gel fraction of the product is generally modelled in the chain crosslinking system. Furthermore a practical model correlating the gel fraction of the product with the polymerization conditions is developed in the vinyl chloride - divinyl monomer suspension copolymerization. The model parameter is estimated in the VC - diallyl phthalate copolymerization. The model could correlate well the gel fraction of the product with the polymerization conditions.