Effects of La-Ce mischmetal on primary Si and eutectic Si in hypereutectic Al-24Si alloy were investigated. The stereo-morphologies of primary Si and eutectic Si in un-modified and in La-Ce modified alloys were obtained by different etching and extracting methods. Microstructure and phase constituent of the alloy were observed by SEM （scanning electron microscope） and XRD （X-ray diffraction）. The results reveal that

on the one hand

the La and Ce enrichment in the liquid alloy of Si/Al interface front can inhibit the growth of Si phase

on the other hand

it causes the constitutional supercooling

during the crystallization process of Al-24Si alloy

which makes the Si phase grow in non-faceted mode or so-called rough interface mode

resulting in the morphology change of primary Si phase and eutectic phase. Composition analysis of rare earth compounds in Al-24Si alloy shows that atomic percentage of Si is higher

showing the partly consumption of Si atoms during RE compound generation process

which results in the decrease of Si content in primary Si and eutectic Si phase to effectively restrain the growth of primary Si and eutectic Si phases.