Abstract: In this paper, the atomic thermomigration behavior and interfacial reaction behavior of Cu/Sn-58Bi/Cu micro solder joints under temperature gradient (TG) were explored at average temperatures of 110 ℃ (aging) and 180 ℃ (soldering). During the aging process, the symmetrical growth of the interfacial intermetallic compound (IMC) was clearly observed in Cu/Sn-58Bi/Cu micro solder joints due to the net structure of Bi phase. There were no obvious Bi atoms thermomigration under the TG of 1 000 ℃/cm. While when the TG reached or exceeded 1 300 ℃/cm, Bi atoms migrated from the hot end to the cold end and then significantly segregated near the cold end interface. During the soldering process, with the driven of TG, the migration of Cu atoms from the hot end to the cold end resulting in the asymmetrical growth of IMC between the hot and cold ends. And there were no obvious thermomigration of Bi atoms. In conclusion, the TG had caused the mass Cu atoms and no Bi atoms to migrate from the hot end toward the cold end, resulting in asymmetrical growth of IMC with the solder in liquid-state. However, with the solder in solid-state, there was no obvious thermomigration of Cu and Bi atoms under lower TG and only thermomigration of Bi atoms under higher TG.