Analytic expressions for grafting of polystyrene(PS) onto polybutadiene(PBD) and crosslinking of PBD during thermal bulk polymerization of high impact polystyrene(HIPS) were derived by using moment distribution function based on Peng’s kinetic scheme[1]. The ratio of chain transfer constant for PBD to that for styrene monomer, k₅/k₃, was determined by the linear correlation between grafting efficiency(GE) and ln 1/(1-X)where X was the conversion of styrene. Both the GE and ln 1/(1-X) were determined from experiments. According to this correlation, the GE did not depend on the reaction temperature, which seemed that the temperature dependence was offset in the ratio of chain transfer constant for PBD to that for styrene monomer in this analytic expression. The degree of crosslinking drastically increased at high temperature(≥190℃) and at high conversion such as 95%, with minor modification, these analytic formulas could be used for the polymerization using initiators. Furthermore, it will be useful to design the polymerization process.