The following affecting factors of rubber toughened epoxy resin system were studied: 1. kinds of curing agent used 2. the Mc value of the matrix 3. the bonding force between the dispersed phase and the matrix. Accoring to our experimental results

the average chain length between crosslinks (Mc) is found to be a much more important affecting factor

chemical bonding between the dispersed phase and the matrix is also important. A toughening mechanism of rubber toughened epoxy resin has been proposed. In the material with relatively low cross-linking density

extensive fracture process stripe is formed which is induced by the com-bined stress field near the rubber particles. The chemical bonding between the dispersed phase and the matrix may increase the strength of the local stress field about the rebber particles which is infavor of broading the fracture process area.