The morphology of MgAl2O4 and Al2O3 particles in Al2O3 / Al-1.0 mass% Mg2Si alloy were investigated by transmission electron microscopy (TEM) in order to understand decrease of age hardening by addition of alumina particles. Hardness of composite materials was lower than the matrix alloy, when composite materials were aged at higher 423K. It has been considered that the addition of alumina (Al2O3) particles caused reduction of the age-harden ability by the decrease of magnesium content in the matrix due to the formation of spinels (MgAl2O4) near alumina particles. Spinels indicated special orientation relationships with alumina pacrticles of {1120)(alumina) // {111}(spinel), <1101>(alumina) // <112>(spinel), <1102>(alumina) // <110>(spinel), and {0001}(alumina) // {111}(spinel), <1120>(alumina) //<112>(spinel), <1100>(alumina) //<110>(spinel).