Jointed interpenetrating polymer networks of epoxy resin and poly(dimethylsiloxane) (PDMS) are synthesized using PDMS-alpha,omega-diol as the reactive modifier. We attempt to control the reaction rate and the morphology of the rubber domains by using the acid as the catalyst. According to the scanning electron microscopy results, the morphologies of the acid catalyzed specimens do not show strain-induced crystallization during the fracture mode due to the homogeneity of the molecule structure. The tensile strengths and moduli decrease when the reaction rate is increased. However, the acid contents have no effect on the fracture energy.