ABSTRACT β-Lactoglobulin A solution at pH 6.4 was heated to 180 °C at the rate of 6 °C/min. By differential scanning calorimetry two independent endothermic peaks were observed. The first peak appeared below 100 °C is corresponded to the thermal denaturation of protein. This conformational change led to the aggregation and polymerization of molecules through disulfide linkage, particularly observed above 100 °C. The second endothermic peak appeared around 150 °C, which was brought by the decomposition of molecules as judged from electrophoresis. Up to 100 °C the viscosity of β-lactoglobulin A solution increased by heating, while the viscosity was reduced beyond 113 °C, due to change in the size of aggregate and decomposition of β-lactoglobulin A molecules.