The oxidation behaviors of a Ti3AlC2/20(vol%)TiB2 composite, synthesized by means of in situ reactions of Ti, Al, graphite, and B4C powder mixtures under hot pressing, have been investigated at 1000 degrees-1400 degrees C in air. The Ti3AlC2/20TiB(2) composite followed the logarithmic oxidation law, and had a lower oxidation rate than the matrix Ti3AlC2. During the oxidation at 1000 degrees-1400 degrees C, a continuous Al2O3 formed on the composite, meanwhile other different oxides formed on the top surface of the Al2O3 layer, depending on the oxidation temperature: discontinuous alpha-TiO2 at below 1200 degrees C, mixture of Al2TiO5 and TiO2 at 1300 degrees C, and continuous Al2TiO5 at 1400 degrees C. The selective oxidation of Al occurred at the oxide/substrate interface through inward diffusion of oxygen, and TiO2 overgrew the Al2O3 layer to form coarse grains through outward diffusion of titanium ions. When the temperature was above 1280 degrees C, Al2TiO5 appeared due to a eutectic reaction between TiO2 and Al2O3. The incorporation of TiB2 promoted the generation of great numbers of voids at the Al2O3/substrate interface and in the Al2O3 inner layer, which is proposed as being the main reason for the Ti3AlC2/20TiB(2) composite to follow a logarithmic oxidation law.