Effects of cold rolling on the aging kinetics in an Al–Mg–Si alloy have been examined mainly by means of differential scanning calorimetry and transmission electron microscopy. The first exothermic reaction detected at temperatures around 350 K in the cold rolled specimen is probably due to the migration of quenched-in vacancies to the dislocations introduced by cold rolling and the molar heat for this reaction is decreased by cold rolling. The subsequent diffuse exothermic reaction may arise from the segregation of solute atoms to the dislocations, Cottrell’s atmospheres being formed. The needlelike precipitates formed in the non-recrystallized matrix of the cold-rolled specimens, and this reaction was displaced to lower temperature side by cold rolling. The molar heat and the activation energy for the needle formation did not vary with reduction, but the activation entropy increased.