The effects of modification by antimony on Al－Si cast alloys have been extensively used, but the modification mechanism has not been well accepted. 0.2wt%Sb has perfect modification effects at the existence of of Mg, while much enough Sb for example 1wt % has effects on modification as well without Mg. The cooling process of alloys and the existence manner of elements in cast Al-Si alloy of different content of Sb is studied in this paper. Viewed from the results of DSC thermal analysis, crystallization supercooling increases, nucleation supercooling reduces and eutectic latent heat increases when the content of Sb reaches 0.2wt %. HCombiningH the existence manner of elements in cast Al-Si alloy analyzed by energy - dispersive spectrometry, it is found that the actual modification agent is the clustered Sb in free state. Mg can intensify the modification effects of Sb because Mg added to alloy decreases the solubility of Sb, which allow enough Sb left over to effect modification of alloy.