The atomic and electronic structures of a Cu grain boundary with segregated Zn have been calculated by the first-principles method based on density functional theory and the effect of Zn segregation on Cu grain boundary is also analyzed. Results show that Zn is segregated to the Cu grain boundary in the way of substitution. Both Cu and Zn have the similar bonding characteristic with their neighbors, which are metallic bonds with a little covalentlike component. The Cu grain boundary with segregated Zn has strengthened the cohesion across the boundary slightly as compared with the clean Cu grain boundary because a small amount of charge accumulation is found between Zn and near neighboring Cu atoms due to the segregation of Zn. Grain boundary with segregated Zn would be fractured between Zn and Cu atoms because the d orbit of Zn is much more localized during the tensile test, resultsing in the weakness of Zn–Cu bond.