Effects of external heating on flame stability were investigated experimentally in a micro porous combustor. The flame stability limits, extinction process, and low-temperature oxidation were studied at various external thermal inputs, from 0.32 W to 0.94 W, and corresponding recirculation efficiency is 3.58-10.53%. Results show that external heating can extend flame stability limits obviously in micro porous combustor. Flame stability limits are 0.343 and 4.285 when external thermal input is 0.94 W, which is wider than that at standard condition. Porous medium temperature decreases gradually after stopping external heating, which causes flame to oscillate and extinguish. Time between stopping external heating and flame extinction increases from 15.7 s to 24.2 s with heating power from 0.32 W to 0.94 W. When temperature of gas mixtures is lower than 233.2 degrees C, n-heptane hardly reacts with oxygen in porous medium. When temperature is higher than 233.2 degrees C, low-temperature oxidation reaction of n-heptane occurs in porous medium and gas temperature increases. Temperature rise is 5.8 degrees C when gas mixtures temperature is 324.9 degrees C.