The change of microstructure of Al-7wt%Si alloy was examined by using a induction motor of 3-phases and 2-poles in semi-solid electromagnectically stirring process. The size and degree of sphericity of primary solid particles were observed by means of change of cooling rate. At a certain cooling rate, the size of primary solid particles increased and the sphericity became perfect, as the fraction of solid increased. The average size of primary solid particles was 43㎛ when the fraction of solid was 0.3 and at 600℃, input voltage 140V, cooling rate 2.5℃/s. It was shown that the size of primary solid particles was proportional to local solidification time.