<p>  The effect of applying vibration to molten metal on grain refinement of aluminum alloy was investigated using a vibrating mold. Grains were refined by vibrating a metal mold during solidification. The more the frequency or half amplitude was increased, the more the average size of grain decreased. Because each frequency and half amplitude affected grain refinement, it is thought that the average grain size can be summarized by the excitation force composed of them. The grain size decreased with increasing excitation force. When the frequency and half amplitude were changed respectively under constant excitation force, the half amplitude was more effective for grain refinement. When the half amplitude was increased under constant excitation force, the area fraction of equiaxed grain increased and the length of columnar crystal shortened. In order to clarify the mechanism of grain refinement, the starting time of vibration was changed during solidification. When vibration was applied to the mold after pouring, both the size of equiaxed grain and length of columnar crystals changed little regardless of the vibration time. On the other hand, when vibration was applied to the mold 15 seconds after the pouring, the length of columnar crystal increased and the size of equiaxed grain increased considerably. From these results, it was clear that starting vibration before solidification is important for grain refinement.</p>