Microstructure and mechanical properties of ternary PP/POE/nano-CaCO3 composites were investigated. Two different phase structures were obtained mainly by adjusting processing sequence. The one-step process led to the isolated distribution of elastomer and CaCO3 particles in PP matrix, while the two-step one attained an encapsulated microstructure. In comparison with binary blends of PP/POE or pure PP, toughness of the ternary composites was significantly increased. Meanwhile, their stiffness and tensile strength kept nearly unchanged or slightly enhanced, implying that there is a synergistic effect between nano-CaCO3 and POE components.