The structural control of Al-Si alloy, which was not studied among various electromagnetic processing of materials, was considered applying the alternating current and direct current magnetic flux density. The main aim of the present study is to investigate the effects of electromagnetic vibration on the control of the size of primary Si phase in order to develop a new process of structural control in Al-Si alloy. If the current density conducted for making high frequency electromagnetic vibration (EMV) (≥ 500Hz), the size of primary Si phase goes to small. If it conducted for making low frequency EMV (≤ 200Hz), the size of primary Si phase goes to large. This phenomenon considered to be related the collision, agglomeration and diffusion of silicon atoms.