High Impact Polystyrene (HIPS) polymers varying widely in the degree of graft, size of the rubber particles and volume fraction of the rubber phase have been tested in uniaxial tension. From these tests, it is shown that the yield stress and tensile modulus decrease as the degree of graft increases up to about 120%, and then increase. Increasing the rubber volume fraction and the size of the rubber particles also reduces the yield stress. A dynamic test is conducted and it is verified that the glass transition temperature of the rubber phase increases with increasing the degree of graft. In addition to this, the maximum tan delta increases with increasing the degree of graft up to about 120%, and then decreases. It is also shown that the yield stress and tensile modulus of HIPS have a relationship with the dynamic behavior of the rubber phase, i.e. the yield stress and tensile modulus decrease almost linearly as the maximum tan delta increases. (C) 2000