We have developed the refinement process of the microstructure of metallic materials by imposition of electromagnetic vibration force during solidification. This process is effective for both wrought magnesium alloys and cast ones. By simultaneous imposition of a static magnetic field of 10 Tesla under an alternative electric current of 60A, the average grain sizes of the AZ31B wrought alloy and the AZ91D cast alloy were obtained about 50 micron in both alloys. The grain size was affected by electric current frequency and had the minimum value at the special electric current frequncy of 500 to 2000 Hz and 900 Hz for wrought alloy and cast alloy, respectively. From experimental results, we suggested the mechanism of refinement of microstructure during solidification by imposition of electromagnetic vibration force. The cavitation phenomenon in liquid phase during electromagnetic vibration was effective to break down th esolid phase. And also the difference of electric conductivity between the solid phase and the liquid one brought vigorous vibration of the solid phase. Then the solid phase was suppressed its growth.