In this study, the effect of 3 mass%Cu additions on microstructure formation and Cu segregation in 800 MPa grade ductile cast iron during solidification was investigated. The calculated phase diagram showed that after the addition of 3 mass%Cu, the Cu phase with a negligible amount appeared below 1000°C, and most Cu was included in the matrix. Based on optical microstructure, after the addition of 3 mass%Cu, the size of graphite nodules became finer, and the microstructure rarely had an area with α-ferrite. Image analysis showed that the fraction of pearlite increased significantly, indicating that Cu greatly promoted the formation of pearlite. Compositional analysis by scanning electron microscopy indicated that the pearlitic area also contained approximately 3 mass%Cu, which corresponds to those of primary and secondary austenite calculated. A small and bright phase particle containing a large amount of Cu was observed at the interface of graphite and matrix.