It is known that thermal analysis method can record the time-temperature curves of aspecimen during solidification. In this paper
according to the thermal analysis curves and manyexperiments data
a compound rule of Na-Mg modification for Al-St Alloy has been obtained
simultaneously mathematical calculating formulas between △T and σb is put forward. Finally calculated numbers had been compared with the experimental results.the facts have proved that thismethod is very useful to production.