The thermal conductivities of a medium density polyethylene composites filled separately with two different thermal conductive fillers including graphite, cuprum, and aluminum oxide (Al2O3), an epoxy composites filled respectively with two different thermal conductive fillers including silicon nitride, aluminum hydroxide, Al2O3 and aluminum nitride, and a polypropylene composites filled with aluminum hydroxide and magnesium hydroxide were estimated using a thermal conductivity equation of polymer multiphase composites. This equation was based on a new heat transfer model, and the parameters were easily determined. It was found that the estimated thermal conductivities of the three composites systems were approximately close to the experimental measured data reported in literature. In addition, the predictions were roughly close to the estimations from the Agari model. POLYM. ENG. SCI., 57:965-972, 2017. (c) 2016 Society of Plastics Engineers