Adopting the undercooling technique combined molten glass denucleating with cyclical superheating, the solidification microstructures of undercooled and hypercooled Fe-B eutectic alloy melts were investigated by the thermal analysis of the cooling curves. The results show that the variations of cooling curves of alloy melts reveal the undercooling extent of melts, i.e. according to the analysis of the characteristic parameters in cooling curves the achievement of undercooled and hypercooled microstructures can be determined directly. And the corresponding relation between the nucleation, distribution, grain size as well as eutectic morphology of the primary phase and the undercooling degree can also be reflected by the cooling curves.