Composite materials can be promising to decouple electron and phonon transport for improved thermoelectric properties. Herein, the synergistic effect of the particle size of the dispersed phase and the interface thermal resistance (Rint) between the phases on the phonon thermal conductivity (κph) of the (1-x)La0.95Sr0.05Co0.95Mn0.05O3/(x)WC thermoelectric composite is demonstrated. The embedded WC, owing to its high electrical conductivity (σ), does improve σ of the composite. A high acoustic impedance mismatch between the phases results in a large Rint and lowers κph. Change in κph is further explained using theoretical analysis of Bruggeman asymmetrical model. The correlation between Rint and Kapitza radius is discussed to analyze the reduced κph. Synergistic effect of improved σ and lowered κph result in improved thermoelectric figure of merit (zT) of 0.20 at 463 K for (1-x)La0.95Sr0.05Co0.95Mn0.05O3/(x)WC composite. This study shows promise to design thermoelectric composites with the desired κph considering the elastic properties between the phases.