The precipitation behaviour of dispersoids in Direct Chill (DC)- cast 3xxx alloys with varying compositions of manganese and silicon has been studied during isothermal annealing at low temperatures. The evolution of density, size and volume fraction of the dispersoids has been quantitatively studied by Transmission Electron Microscopy (TEM) and electrical conductivity measurements. Moreover the influence of precipitation of dispersoids on hardness and tensile strength of the alloys is systematically investigated. A clear dispersion hardening effect from dispersoids is revealed and the hardening effect increases with increasing Mn and Si contents in the alloys. The hardening effect from dispersoids is discussed in light of the Orowan mechanism.