This work suggests a modeling method based on micromechanical models such as Pukanszky one to determine the interphase properties in ternary polymer nanocomposites containing two nanofillers. The developed models can calculate the volume fraction, thickness and strength of interphases between nanoparticles and polymer matrix using the experimental data of Young’s modulus and yield strength. The properties of interphases are calculated and discussed for polypropylene/montmorillonite (Mt)/CaCO3 ternary polymer nanocomposites at different nanofiller contents (2, 4 and 6 wt % of Mt and 2, 8, 14 and 20 wt % of CaCO3). The developed models demonstrate that the thickness of interphase differently depends on nanofiller size and volume fractions of nanofiller and interphase. In addition, it is shown that the strength of interphase directly correlates to the interfacial interaction/adhesion between nanoparticles and polymer matrix.