Tensile properties of cross-linked rubber such as tensile strength, tensile elongation and tearing energy change 2 〜 3 orders of magnitude according to the variation of temperature and velocity within rubbery state. These phenomena result from the dominant contribution of the viscosity to the rubber elasticity. This seems to indicate that a phase separation between elastic and viscous parts takes place, and thus macroscopically bi-continuous structures are formed in real cross-linked rubber.