Electromagnetic stirring was applied as a treatment method of semi-solid state processing. The study on the change of the microstructure of Al-4.5wt%Cu alloy was conducted through the size of primary solid particle and its sphericity in various stirring conditions. First, we produced a rotating magnetic field type, electromagnetic stirring apparatus having 3-phases, 2-poles and common frequency of 60㎐(inner diameter of core : 191㎜;height of core : 225㎜). The apparatus specified that the magnetic flux density was about 0.05T at the center when maximum power was loaded. Microstructures obtained by electromagnetic stirring treatment were mixed morphology which consisted of primary solid particles and fine eutectic phases caused by the quenching effect of remaining liquid phase. The shapes of primary solid particle were rosette type or spheroid type and the effect of electromagnetic stirring on molten alloy as the stirring method of semisolid state processing was confirmed.