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Thermal reaction and microstructure evolution of porous Ti3AlC2 ceramics in a graphite bed (65% N2 and 35% CO) between 1100 and 1500°C were investigated in this work. The main products of Ti3AlC2 in a graphite bed were titanium carbide and alumina. At 1100°C, a thin alumina layer formed on the Ti3AlC2 particles due to the outward diffusion of slight aluminum. As the temperature increased to 1300°C, more aluminum diffused outward from the Ti3AlC2 particles, and a partial of Ti3AlC2 transformed into titanium carbide. Consequently, the alumina layer formed on the surface of particles got thicker, and defects occurred inside the particles due to the Kirkendall effect. When the temperature was 1500°C, Ti3AlC2 was transformed entirely into alumina and titanium carbide, and visible pores were left inside the particles. The formation mechanism of TiC/Al2O3 composite from porous Ti3AlC2 ceramics in a graphite bed was proposed in detail.