The precipitation of dispersoids in DC cast AA3003 billet during heating and homogenization at 600degreesC was studied by the means of Transmission Electron Microscopy, electrical conductivity measurement and image analysis. During heating (50degreesC/h), the dispersoids, which mainly nucleate on the dislocations begin to precipitate at about 300degreesC. The type of dispersoids is determined to be alpha-AlMnFeSi phase. The density of dispersoids increases with temperature and achieves a maximum at about 400degreesC and then decreases with temperature and homogenizing time. The average size of the dispersoids increases during heating and homogenization. The size evolution is controlled by the nucleation, growth and coarsening. The volume fraction of dispersoids shows the same trends as the evolution of the electrical conductivity of the alloy during heating.