The concept of effective crosslinking density proposed by Scanlan is extended to heterogeneously crosslinking polymer. A kinetic-statistical model of effective crosslinking density of the gel is developed in free-radical heterogeneously crosslinking polymerization. The model is applied to the vinyl chloride (VC)/dially phthalate (DAP) suspension process. It is concluded that the model successfully predicts the development of effective crosslinking density of the gel. It is suggested that the degree of swelling of the gel is determined by its effective crosslinking density. However, when the crosslinking density is high, the physical entanglements would greatly influence the degree of swelling.