Plastic wastes have a potential to play a significant role in the reduction of iron oxides by supplying reducing gases, e.g., H₂ and CO, through pyrolysis. However, they are difficult to use as reducing agents because the thermal degradation temperatures of plastics are significantly lower than the reduction temperatures of iron oxides. In this study, the reduction mechanism of iron oxides by carbon and hydrogen obtained from polyethylene in the composite at lower temperature was studied in order to attain a high utilization ratio of polyethylene to the reduction. The reduction experiments were carried out under an Ar–5%N₂ gas flow with a heating rate of 0.33 K/s, and the gases formed during reduction were continuously analyzed. The reduction degree of hematite was calculated using the concentration of these gases. The peak temperatures of gas generation rate for the polyethylene–graphite–hematite composite were obtained at 770, 1070, and 1370 K. The peaks at 770 and 1070 K originated from the reaction of polyethylene, and the peak at 1370 K originated from that of graphite. Larger specific surface area of the iron ore leads to fast reduction rate of the ore by polyethylene. Therefore, high utilization rate of carbon derived from polyethylene was obtained when using iron ore with a large specific surface area, such as goethite ore.