Thermal expansion behavior in a stoichiometric AuCu alloy was investigated from the viewpoint of a phase transition. Thermal expansion measurements were conducted from room temperature to 873 K at various heating and cooling rates. The crystal structures and phase transitions were analyzed by X-ray diffractometry and differential scanning calorimetry, respectively. The cast sample showed a disordered face-centered cubic (fcc) structure, which changed into a face-centered tetragonal structure AuCu I or mixed structure of fcc and AuCu I after thermal expansion measurement. In the thermal expansion curves, the break points corresponded to order-disorder transition. During the heating process, three break points corresponding to three different ordering stages were observed. The activation energy for each stage was obtained using the Kissinger method. It is believed that the first and second stages are ascribable to the migration of excess vacancies induced by quenching, and that the third is predominantly due to self-diffusion.