An experimental study of radiation-modified blends of high-density polyethylene (PE) and ethylene propylene diene rubber (EPDM) exposed at constant magnetic field (MF) with induction B equal to 1.0, 1.4 and 1.8 T is presented. Materials have been irradiated with accelerated electrons with absorbed doses equal to 100 and 150 kGy. The main focus of the research is devoted on deformation (elastic and viscoelastic) properties of polymer materials. The gained data show effect of the absorbed dose and magnetic field induction on the modulus of elasticity E and the creep of the investigated polymer composites under the influence of constant tension. Decrement of elastic modulus and increase of total deformation of unirradiated polymer composites has been observed with increase of the induction B. The effect is reduced by increase of the absorbed dose of electron beam radiation to 150 kGy. The value of E remains almost constant for cross-linked polyethylene, in contrast to the non-irradiated samples, at induction in the range of 0.5 to 1.8 T. Radiation modification and increment of the EPDM content of EPDM minimizes the effect of the magnetic field on elastic properties of materials. Changes of crystallinity and melting temperatures of polyethylene in composites have been investigated to better explain the effect of cross-linking and the compatibility of both the components in investigated materials.