Multiwall carbon nanotubes（MWNTs） reinforced aluminum composites were synthesized by hot press sintering. The thermal stability of MWNTs in the composite was investigated by transmission electron microscopy（TEM）. The results show that the impurities drop from 2% to 0.11% after the MWNTs are soaked with 95%H2SO4 and 66%HNO4 acids in an ultrasonator and washed with distilled water. In hot pressed composites, MWNTs distribute between aluminum powders and do not react with aluminum. When the composite was held at 800℃ for 1h, MWNTs react with aluminum and form Al4C3 phase, which are small polygons and distribute in the aluminum grains. Although Al4C3 phase still forms when the composite was held at 1000℃ for 1h, its morphology changes to a continuous band and distributes along the origin aluminum powder boundaries. So MWNTs are unstable in aluminum matrix composites that are kept above the liquidus temperature of aluminum.
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