The rotating impeller degassing technique was used for degassing of A356 scrap melt which had been protected and purified with an RE-containing JDN-I flux. Results indicated that the modification of eutectic silicon was faded and the mechanical properties of the alloy became worse after rotating impeller degassing and that the tensile strength and elongation of the scrap A356 alloy which was treated with rotating impeller degassing system after having been refined by JDN-I flux decreased by 5.6 and 23.3%, respectively, compared with those of being refined merely with the flux. The mechanism is that the rotating impeller degassing increases the oxygen content and decreases the hydrogen content of the melt, consuming more rare earth from the RE-containing lux. Thus, the content of rare earth that has effect on the modification of eutectic silicon is lower than the critical value. At the same time, the oxide inclusions in the melt increase leading to a drop of mechanical properties of the alloy.