A study on the increase of the flexibility and decrease of the internal stress of cured epoxy resins by addition of polysiloxane rubber particles grafted with and without glycidyl methacrylate (GMA) or methyl methacrylate (MMA) is reported. Polysiloxane latex was obtained by cycling a mixture of polysiloxane containing a vinyl group and a =Si-H group, respectively, emulsifier, and water in a homogenizer. Graft. polymerization of GMA (or MMA) on these rubber particles was performed in the latex under gamma-ray irradiation of Co-60. Th, rubber particles were used to improve the flexibility of the epoxy resin. The particles grafted with GMA showed a homogeneous distribution in the matrix, better inducing and terminating craze. Comparisons were made among the particles grafted with MMA and various MMA-GMA or a series of GMA; the results indicated that the particles grafted with GMA (graft ratio = 10%) showed the greatest increase of elongation and decrease of internal stress. The factors influencing the toughness of the epoxy resin matrix are discussed.