Abstract: By using of electrolytic pure iron, the specimen containing different heterogeneous nucleus and contents were made in an Mo wire furnace. The DTA method was employed to measure the undercooling of liquid iron during solidification. Equations to calculate the change of Gibbs free energy of solid and liquid iron in solidification were reviewed theoretically. Calculations were performed to obtain Gibbs free energy difference between solid and liquid, critical radius of nucleus and the activation energy in solidification.