This work presents design and performance characteristics of large-size ceramic and metal sonotrodes for ultrasonic treatment of molten aluminum alloys in industrial and pilot scale casters. ANSYS software was used to predict vibration characteristics and to design sonotrodes of various shapes made of silicon nitride base ceramics or Nb-based refractory metals. The design concept is based on requirements that users actually need to apply the sonotrodes on industrial scale, particularly high vibration amplitudes and large diameters of working tip. Both these parameters were found to be determined by the sonotrode shape, connection reliability and material properties. A number of sonotrodes was fabricated and examined for their ability to treat aluminum melt in DC casting process in order to refine the solidification structure. Although the ability to refine the metal structure was about the same for sonotrodes of both types, the ceramics barbell sonotrodes demonstrated much better performance with respect to acoustical and chemical stability, especially at high temperatures. On the other hand, the metal sonotrodes were highly susceptible to cavitation erosion.