A new kinetic theory of tensile strength for polymer networks has been proposed. It is based on the Eyring chemical activation rate theory and the molecular theory of rubber elasticity at large deformations. The coefficient of overstress of a polymer chain is derived by combining the deformation behavior

stress concentration of a polymer chain and the chain orientation as a united event of polymer failure. A kinetic equation for bond rupture of over-stressed chain is established by taking the coefficient of overstress as a variable. After integration and simplification a new equation of state for ultimate fracture is obtained. It is successful in reating the ultimate strength and extensibility to the parameters of structure

the temperature and the extension rate. The failures of four different polymer networks were studied by uniaxial extension method. These experimental data of ultimate properties were treated with the proposed theory. The results show that the proposed theory can predict the failure behavior of polymer networks and provide a theoretical foundation for the construction of failure envelop.