The in situ composites with the reinforcement volume fraction of 30 vol.% and the C/ZrO 2 mole ratio of 0, 0.5 and 1.0 have been fabricated by using exothermic dispersion synthesis in an Al-ZrO 2 -C system. The reaction mechanism and mechanical properties of the composites have also been studied. When the reinforcement volume fraction of the composites is 30 vol.% and the C/ZrO 2 mole ratio is zero, the Al first reacts with ZrO 2 to produce the ␣-Al 2 O 3 particles and the active Zr atoms, and then the Zr atoms react with Al to form the Al 3 Zr blocks, which are distributed uniformly throughout the aluminum matrix. The ultimate tensile strength and elongation of the composites at room temperature are 215.2 MPa and 3.0%, respectively. The fracture mechanism of the composite can be characterized by a crack nucleus initiating in the Al 3 Zr blocks and then propagating to the interface because of the poor properties of Al 3 Zr. With increasing the C/ZrO 2 mole ratios, the ZrC is formed previous to the Al 3 Zr due to its lower Gibbs free energy, and its formation peak becomes bigger in the DSC curve. The amount of the Al 3 Zr blocks decreases, which leads to the improvement in the tensile properties of the composites. When the C/ZrO 2 mole ratio is up to 1, the Al 3 Zr blocks have almost disappeared in the composites. The reinforcements are composed of ␣-Al 2 O 3 and ZrC. At the same time, the ultimate tensile strength and elongation increase to 245.4 MPa and 8.0%, respectively. The tensile fracture surface is composed of fine ductile dimples.