In the process of semi-solid slurry preparation with direct ultrasonic vibration (UV) by dipping the horn into the melt,one of the questions is whether the gas content in the melt would be increased or not by the cavitation effect of ultrasonic vibration.By application of quantitative gas content measurement technique,this paper investigated the effect of the ultrasonic vibration on the gas content of both the melt and the semi-solid slurry of Al-Si alloys,and the variations of the gas contents in two kinds of aluminum alloys,i.e.,A356 alloy and Al-20Si-2Cu-1Ni-0.6RE alloy (Al-20Si for short).The results show that ultrasonic vibration has an obvious degassing effect on the molten melt,especially on the semi-solid slurry of Al-Si alloy which is below the liquidus temperature by less than 20 ℃.The ultrasonic degassing efficiency of the A356 alloy decreases with the reduction of the initial gas content in the melt,and it is nearly unchanged for the Al-20Si alloy.The gas content of both alloys decreases when the ultrasonic vibration time is increased.The best vibration time for Al-20Si alloy at the liquid temperature of 710 ℃ and semi-solid temperature of 680 ℃ is 60 s and 90 s,respectively; and the degassing efficiency is 48％ and 35％,respectively.The mechanism of ultrasonic degassing effect is discussed.