A new technique for the measurement of the solid Si/liquid Al-Si eutectic interfacial free energy Υgb and ΥSi-L. Pure Al or al with added Sr and pure Si including a straight grain boundary were used for producing a bimetallic sample. A decrease in the sample temperature was observed because of the latent heat of Al while the sample was held at 1023K. At this time, the sample temperature started to cool from 1023K to 863K with a period shorter than 60s, and then it was held for 5h at 863K, only a little above the eutectic temperature. The equilibrium grain boundary groove was formed at the solid/liquid interface. The dihedral angle for Al-Si-Sr alloy is lower than that for Al-Si alloy. This fact indicates that ΥSi-L. is decreased by the addition of Sr.