The simultaneous effect of the incorporation of flax fibers and terpolymer ethylene-propylene diene rubber (EPDM) on the properties of ternary composites with a PP matrix is investigated in this study. In order to evaluate the effects of the matrix composition and fiber percentage on the properties of the composite, an experimental design based on a Doehlert Uniform Net was employed. First, the processing conditions of these ternary composites were analyzed through a rheological study, where a noticeable increase of the viscosity upon the incorporation of the fiber and elastomer was observed. Then, the thermal behavior of the composites was studied with dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC). The results indicate a strong nucleation ability of the elastomer, showing a considerable increase of the rate of crystallization. These results concur with those obtained with an optical microscope, where it was observed that a higher number of active nuclei were formed in the presence of the elastomer. A previous characterization of the flax fibers used in this study has been also carried out by measuring the length and diameter distribution as well as the mechanical and thermal properties.