The positron annihilation lifetime (PAL) technique has been applied to investigate the behavior of free volume hole in carbon black loaded ethylene propylene diene monomer (EPDM) composite as a function of the wheat hay concentration from 10 to 50 phr. The macroscopic studies including stress-strain, hardness and swelling behavior were measured too. An attempt is done to establish a correlation between the macroscopic measurements and the microscopic properties of the free volume holes. The ortho-positronium (o-Ps) parameters show a compact phase around 20 phr concentration of wheat hay. Such a phase is thought to be responsible for the maximum values of hardness, rupture stress, rupture strain and the minimum in the degree of swelling.