The effect of overheating on the undercooling behavior of a hypoeutectic Co-B alloy has been studied by molten glass fluxing combined with thermal cycling method. Results indicate that the undercooling of Co83B17 alloy is greatly affected by the overheating temperature and a critical threshold temperature is determined. The existence of the turning point is thought to be related with the structure transitions inside the overheated liquid which is confirmed by in-situ high temperature magnetic measurement. (C) 2014 Elsevier B.V. All rights reserved.