The energy density of chip is becoming increasingly higher with the power electronic devices developing toward miniaturization, high power and integration, which will lead a higher operating temperature. However, the traditional Sn-based soldering process fails to meet the elevated temperature. Transient liquid phase (TLP) soldering, which can form high-melting-point joints at relatively low temperatures, has been proven to be a promising bonding method for solving this technological challenge. Nevertheless, a common drawback for TLP soldering is that it will consume a very long time for the complete formation of intermetallic joints, up to tens of minutes, which will lead extra thermal stress and seriously negative effects on the reliability of packaging systems. Recently, this technological puzzle has been proven to be solved by a novel ultrasonic-assisted TLP soldering process, in which the ultrarapid formation of complete intermetallic joints was achieved due to the accelerated diffusion of Cu from the substrates into the molten Sn interlayer under the complex sonochemical effects of acoustic field on the interfacial reaction. In this study, the microstructure and mechanical properties of complete Cu-Sn intermetallic joints ultrarapidly formed by ultrasonic-assisted TLP soldering process were investigated. The sandwich Cu/Sn/Cu system was placed on the heating platform, and then the ultrasonic vibrations and the bonding force were applied on it. The horizontal ultrasonic frequency, pressure, power, bonding temperature and time were fixed as 20 kHz, 0.5 MPa, 300 W, 250 ℃ and 5 s. In summary, the complete intermetallic joints composed of Cu₆Sn₅ interlayer with a thickness about 15 μm and Cu₃Sn boundary layers with a thickness about 1 μm were ultrarapidly formed by ultrasonic-assisted TLP soldering process. The formed Cu₆Sn₅ grains were remarkably refined to be with an average grain size less than 5 μm. Compared with the intermatllic joints formed by traditional TLP soldering process, the resulted intermetallic joints performed more uniform mechanical properties with elastic modulus and hardness of about 123 GPa and 6.0 GPa respectively, as well as a higher reliability with a shear strength of 60 MPa.