The reaction mechanism of an Al-TiO2 system is discussed. Thermodynamic analysis indicates that the reaction between Al and TiO2 can occur spontaneously due to the negative Gibbs free energy of the Al-TiO2 reaction system. When the reinforcement volume fraction is 30%, there is an endothermic peak and an exothermic peak in the DSC curve. But when the reinforcement volume fraction increases to 100%, there are two independent exothermic peaks and the height of them increases obviously. With increasing the heating rate, the ignite temperature becomes higher and all the peaks move to the higher temperature direction. The reactions between Al and TiO2 consist of two steps: first, Al reacts with TiO2 to form the stable α-Al2O3 particles and the active Ti atoms; second, the active Ti atoms react with Al to form Al3Ti.