Abstract: A series of experiments were conducted with Al-5.7%Cu alloy to investigate the effect of probe horizontal location on solidification structure and grain-refining efficiency of ultrasonic ingots.The mechanism of grain refinement in ultrasonic field was discussed.The equiaxed grain zone is the biggest when the probe position was at the centerline of the mould.The equiaxed grain zone is reduced to the probe end when the probe position was nearby the mould wall.Application of ultrasonic vibration resulted in forming acoustic cavitation,leading to supercooling of micro-area and dynamic nucleation.The acoustic streams make breaking-up of solidifying dendritic crystal and supplied germ nucleus,therefore,the solidification structure of ingots get refined.It is a complex dynamic process.