In this work，the coupling graft of polymers is treated by means of kinetics．The expression of the molecular weight distribution of grafting copolymer is a bi-dimen-sional distribution function with two variables n and m．where n is defined as the total degree of polymerization of the branches and m represents the degree of polymeriza-tion of back-bone．On the basis of this，all of the molecular parameters，such as the number and weight average molecular weights，the grafting distribution，the grafting degree and the grafting success，etc.，can be derived． If the termination of the hying prepolymer is present during the grafting reaction process，parameter λ introduced in this paper must be regarded as the grafting efficiency，and some equations obtained must be revised．The revised equations are still applicable for some other types of gra-fting reaction．For instance，these theoretical equations are appropriate also for the free radical copolymerization of styrene on polybutadiene．