The effect of liquid-liquid structural transition (LLST) on the undercoolability of liquid Co-Sn eutectic alloy has been investigated via cyclic overheating and undercooling. Results show that the stable undercooling increases considerably when the melt is overheated above a critical overheating temperature. The LIST is evidenced by differential scanning calorimetry (DSC) experiment and is thought to be the major factor for the sharp increase of undercooling at the critical point. The effect of LLST on undercooling is analyzed quantitatively within classical nucleation theory. (C) 2015 Elsevier B.V. All rights reserved.