A probe consisting of a cartridge heater was inserted into melted polypropylene composites (PP/mica = 5, 10, and 20 wt%) flowing in a die having a diameter of 10 mm, and the effects of mica on the heat transfer coefficient and the heat transfer behavior from the surface of the probe to the flow compound material were examined experimentally. As a result, the heat transfer coefficient of the PP/mica composites increases 15-20% depending on the flow velocity and filler content. In general, the Prandtl number (Pr) for high-viscosity (eta) melted polymer is greater than one (Pr > 1), and the Prandtl number for polypropylene composites increases as the viscosity increases and decreases depending on the flow velocity and the temperature. The relationship between Pr and eta is directly proportional, and the beta value, which is given by the slope of the Pr - eta diagram, is a constant that shows the viscosity dependency of the Prandtl number. The beta value can be determined by the ratio of the specific heat to the thermal conductivity. The Nusselt number depends on the mica content of the filler and increases gradually.