Average number of radicals per polymer particle was studied theoretically, considering the process for radical to escape from particles and the retardation process in or on polymer particles. Moreover, the physical meaning of the rate coefficient of the former process was analyzed theoretically, on the assumption that radical of lower molecular weight was able to escape from particles. The validity of the above analysis was clarified, using the published data for several monomers. For the average number of radicals, the solubility of monomer in water and the reactivity of monomer are both important factors.