Aging process of Al–1.0 mass% Mg₂Si alloy composite materials with 4 vol% and 8 vol% Al₂O₃ particles was investigated by Vickers micro-hardness measurement and transmission electron microscope observation. When Al₂O₃ particles are dispersed into an Al–Mg–Si alloy, the age-hardenability was obviously lowered above at 473 K aging. The rectangular shaped spinels were observed at the interface between Al₂O₃ particles and the matrix. The overaged composite material with 4 vol% Al₂O₃ particles incluedes various kinds of metastable phase precipitates: namely β′ phase, the TYPE–B and TYPE–C precipitates of which crystal structures are different each other. Fraction of the three kinds of precipitates are the same as in the mother alloy containing 0.2 mass% excess Si. The composite material with 8 vol% Al₂O₃ particles contains the TYPE–A, TYPE–B and TYPE–C precipitates, but is free from β′ phase precipitates. The existence of those precipitates has correspondingly been observed in the mother alloy containing 0.4 mass% excess Si.