Positron annihilation lifetime measurements were performed on pure polypropylene (PP), ethylene-propylene-diene monomer (EPDM) rubber, and their blends PP/EPDM with a series of EPDM volume fraction phi (= 10 - 40%). A numerical Laplace inversion technique (i.e., CONTIN algorithm) was employed to obtain the probability distribution functions (PDF) of free-volume radius. We observed that, first, the average free-volume radius in PP/EPDM blends is generally same as that in PP and is much smaller than that in EPDM. Second, the standard deviation sigma(R) or the width of the free-volume radius PDF in the blend decreases with phi in the region of phi = 10 - 30%, and it increases when phi increases from 30% to 40%. The difference in the sigma(R) of the blend and the calculated value sigma(R)(c) according to the simple-mixing rule of PP and EPDM is interpreted by the existence of the two-phase interaction (i.e., the residual thermal pressure and shear stress between PP and EPDM phases in the PP/EPDM blends). The correlation between sigma(R), which indicates the interaction of two phases, and the impact strength of PP/EPDM blends was found and discussed.