The thermal expansion behaviors of aluminum borate whisker and Fe3O4 reinforced pure aluminum matrix composite with different thermal treatments were studied. Using transmission electron microscopy (TEM) the microstructures of the composite were investigated. The results indicated that the thermal expansion coefficient of the new composite was low. It can be found that the thermal expansion coefficient of as-cast composite is much lower than that of quenched and annealed composites. On the basis of phase transformation analysis of Fe3O4 particles in the composite, the mechanism of low CTE of the composite was discussed.