This paper discusses the stability of radicals produced under gamma irradiation for phenyl-substituted polysilanes with different backbone structures. Poly(methylphenylsilane) and structural defect-containing phenyl-substituted polysilanes were irradiated by Co-60 gamma-rays in the solid state. Temperature dependence of the EPR signal intensity from the radicals induced by radiolysis was measured. The radicals appeared to be more stable as the induced defect density in the backbone structure was increased, indicating that the structure defects on the polymer backbone may play a role in stabilizing silyl radicals. The migration of unpaired electrons was also observed from chain ends to chain center leading to stable radical species. The estimated-branched structures were less than 3.5% in the linear polysilane obtained by conventional Wurtz coupling condensation.