Semi-Solid Al-5%Mg-3%Zn-1%Mn alloys with or without 0.2%Zr were prepared by the isothermal heating of either cast ingot or extruded billet. The microstructural and tensile properties variations during the isothermal heating, depending on alloy composition and slurry preparation method, were investigated. The grain (globule) size of semi-solid microstructure was not remarkably changed by the slurry preparation method, however it was significantly reduced by the Zr additions. The room temperature tensile elongation of the semi-solid formed alloys was generally higher in case of the extruded billet. T6 heat treatment was not found effective in improving the strength of the semi-solid alloys, unlike the casting alloys.