The effect of ultrasonic vibration on the fluidity and microstructure of cast aluminum alloys (AlSi9 and AlSi18 alloys) with different solidification characteristics was investigated. The results show that ultrasonic vibration can affect the fluidity of alloys in both solidification and hydrodynamics aspects. For AlSi18 alloy without dendrite growing solidification characteristics, the microstructure is almost not influenced by ultrasonic vibration, and the influence of ultrasonic vibration on its fluidity is mainly in hydrodynamics aspects. That is, appropriate ultrasonic vibration can improve fluidity by reducing the flow resistance of the melt, but when the vibration intensity is high enough to induce turbulence in the melt, the turbulence will increase the flow resistance greatly and decrease fluidity. However, for AlSi9 alloy, which obviously has dendrite growing solidification characteristics, ultrasonic vibration can influence solidification by breaking the growing α (Al) dendrite, consequently refining the solidification microstructure. Ultrasonic vibration could then improve the fluidity of AlSi9 alloy not only from the hydrodynamics aspect but also by breaking the dendrite network in the mushy zone to decrease flow resistance.