The prediction and tailoring of thermal conductivity of two-phase composites is essential. In this work a new semi-empirical model was developed, which was derived from the rule of mixtures. Furthermore, a new methodology was developed to determine the thermal conductivity of the fillers and the maximum achievable filler content. To validate the new model, polypropylene-based composites were prepared with different fillers, such as talc, boron-nitride and graphite, with 20, 40 and 60 vol% filler content. The results obtained from the proposed model are in good agreement with the experimental data. Various other theoretical models were also introduced and compared to the experiments, but in most cases those underestimate or overestimate the thermal conductivity of composites.