A synthesized polydimethylsiloxane containing an isocyanate group was used to improve the flexibility and to reduce the internal stress of epoxy resin cured with MDA (4,4＇-methylene dianiline). The effect of polysiloxane content on the curing kinetics of a novolac-type epoxy modified with an isocyanate group was investigated. It was found that the modified epoxy resin showed significant improvement in impact strength. The polysiloxane containing isocyanate groups effectively depressed the internal stress of cured epoxy resins by reducing the flexural modulus and the coefficient of thermal expansion, while the glass transition temperature was increased.