Measurement of between ethylene-propylene rubber (EPR) and polypropylene (PP) is an important research subject in the study of rubber toughened PP. When the ethylene content in EPR is low, the EPR and the PP phases become optically indistinguishable due to their similar refractive indexes, and thus the measurements of interface free energy by conventional methods are impossible. In this study, we devised a new experimental technique that enables measurement of the interface free energy between two polymers having similar refractive indexes. When small amount of inorganic additives are incorporated to the PP phase, interface between PP and EPR phases are clearly seen and the measurements become attainable. Using the suggested method, the interface free energy between EPR and PPs were obtained and presented. Four different PPs were investigated, homo PP and three random PPs that contain small amounts of ethylene unit ranging from 1 to 3 wt%. It was found that the interface free energy decrease as the ethylene content in the PPs increases and the effect of the ethylene content on the interface free energy is unexpectedly large.