The interfacial reactions and diffusion behaviors of Zn atoms in Cu/Sn-9Zn/Ni interconnects during liquid-solid electromigration (L-S EM) under a current density of 5.0×103 A/cm2, 1.0 × 104 A/cm2 and 2.0 × 104 A/cm2 at 230 oC have been in situ studied using synchrotron radiation real-time imaging technology. Zn atoms would directionally diffuse towards the Cu interface under both flowing directions of electrons with the current density of 5.0×103 A/cm2, then taken part in the interfacial reaction, resulting in the thickness of intermetallic compounds (IMC) at Cu interface thicker than that at Ni interface. While when the current density rise to 1.0 × 104 A/cm2 and 2.0 × 104 A/cm2, the reverse polarity effect, evidenced by the continuous growth of intermetallic compound (IMC) layer at the cathode and the thinning of the IMC layer at the anode, was resulted from the abnormal directional migration of Zn atoms toward the cathode in electric field, which was more significant at high current density. Irrespective of the flowing direction of electrons, the consumption of Cu film was obvious while that of Ni film was limited. The dissolution of anode Cu followed a linear relationship with time with current density of 1.0 × 104 A/cm2 and 2.0 × 104 A/cm2 and electrons flowed from the Ni to the Cu, and the consumption rate was magnitude higher at high current density. It is more damaging with electrons flowing from the Ni to the Cu than that from the Cu to the Ni. In addition, based on the electromigration flux Jem and chemical potential gradient flux Jchem the diffusion behavior of Zn and Cu atoms were analyzed.