Simple blends with different viscosity ratios of the components as well as compatibilized blends varying both in type and content of compatibilizers were used to study the relation between interfacial tension and dispersed-phase particle size for PP/EPDM (80/20 wt %) blends in this work. Four compatibilizing systems, poly(ethylene-co-methacrylic acid) ionomers (EMA-I), dicumyl peroxide (DCP), DCP combined with EMA-I, and DCP in combination with trimethylol propane triacrylate (TMPTMA), were used. For blends prepared in an internal mixer, a power law relation was found between capillary number and torque ratio of the blends' components. This relation was used to estimate the interfacial tension for the compatibilized blends. The relation between steady-state torque of the blends as a measure of viscosity and the estimated values of interfacial tension were also investigated.