Abstract: The expandable graphite (EG) was microencapsulated by polyurea, and CuO was doped into the shell of the expandable graphite microcapsule (EG@PO) in order to improve the thermal conductivity of the shell. The microcapsules were characterized by SEM, thermogravimetric analysis and Fourier transform infrared spectroscopy. The effect of EG@PO and ammonium polyphosphate (APP) on the flame retardant properties of natural rubber (NR) was also investigated by limited oxygen index test, vertical burning test, thermogravimetric analysis, cone calorimeter test and thermal conductivity measurement. The results show that when mass of EG@PO is 6 g, the limited oxygen index of EG@PO/NR composite is 28.3%, and the vertical burning of composite reaches to V-0. The residual mass is 27.5% at 600℃. Furthermore, the heat release rate and total heat release of EG@PO/NR composites decrease dramatically to 467.7 kW/m² and 48.4 MJ/m², respectively, which is 49.8% and 25.7% lower than that of the pure NR. At the same time, the thermal conductivity of EG@PO/NR composites increases to 0.266 W/(m·K), because the CuO doped into the shell is beneficial to the heat transfer between the NR matrix and EG.