CuO particles were introduced into liquid Al-2Mg alloy by the vortex method to prepare an Al alloy-MgAl2O4 in situ particle composite, by reaction between CuO particles and the Al-2Mg alloy melt. Pure Cu, MgAl2O4 and MgO particles were detected in the particles extracted from the composites. DTA study showed partial dissolution of Cu in the matrix. Microhardness and hardness of the composites are higher than those of the base alloy. Both microhardness and hardness are higher for the Al-2Mg-2CuO composite than those of the Al-2Mg-5CuO composite. The hard ness of the Al-2Mg-2CuO composite is remarkably high. The increase in microhardness has been attributed to the solid solution hardening effect with Cu as well as to the difference in CTE between the Al matrix and the particles. On the other hand, the improvement in hardness resulted from both solid solution hardening as well as the presence of hard particles such as MgAl2O4 and MgO.