Abstract: It is always assumed that polymerization rate is equal to chain propagation rate while setting up kinetics model of radical polymerization.In the assumption the rate of monomer consumption due to the reaction of primary radicals with monomer is very small and can be ignored,and that is the long-chain assumption.This paper deals with the as sumption in detail.The calculated results indicates that in the case of high temperature and／or high initiator concentration,especially in a starved feed reactor,the long-chain assumption needs correcting so that a more accurate kinetics model of radical polymerization can be set up.