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A native alumina thin film existing on aluminium and its evolution in thermosonic gold wire bonding were characterized using high resolution transmission electron microscopy (HR TEM). It has been found that the partly fracture of this alumina film by the ultrasonic vibration allows the interdiffusion at the Au/Al interface, such that the intermetallic compounds (IMCs) are formed. The IMCs are identified as Au 4 Al and AuAl 2 with a thickness of approximately 300 nm. With a higher ultrasonic energy, IMCs are formed in more contact areas, resulting in a stronger bond. Based on these experimental results, a mechanism of ultrasonic gold wire bonding is proposed, which helps to understand the correlation among ultrasonic energy, interfacial structure and bonding strength.