Severe plastic deformation using the process of equal-channel angular pressing (ECAP) was applied to an age-hardenable Al-Mg-Si alloy containing precipitate particles. Subsequent microstructual changes were examined using conventional transmission electron microscopy and energy-filtered transmission electron microscopy. It is shown that metastable rod-shaped beta' particles are fragmented into round-shaped particles by the severe straining and there is a dissolution of the particles with an increase in the imposed strain. Further aging leads to the formation of cube-shaped particles in sample where the dissolution of particles occurs. It is also shown that the aging is significantly accelerated, and Si particles are formed, if it is undertaken using samples subjected to ECAP.