Crystal structure of Ln2Ni1-xCuxO4+δ (Ln= La, Pr, Nd, Sm and Eu), which are expected as new cathode materials for solid fuel cells, was investigated. Single phase with K2NiF4 structure, so-called T-phase, was obtained for La2Ni1-xCuxO4+δ regardless of Cu content. Single phase with Nd2CuO4 structure, so-called T'-phase, was obtained for Sm2Ni1-xCuxO4+δ and Eu2Ni1-xCuxO4+δ for only 0.95≤x≤1.0 and x=1.0 respectively. No T-phase was obtained for the other composition of Ln2Ni1-xCuxO4+δ (Ln=Sm, Eu). The single T-phase and T'-phase were obtained for Pr2Ni1-xCuxO4+δ with x≤0.4 and x=1.0, respectively. For Pr2Ni1-xCuxO4+δ with x between 0.5 and 0.9, phase separation into T-phase and T'-phase due to miscibility gap was observed, showing similar behavior with Nd2Ni1-xCuxO4+δ except for little smaller x range of the miscibility gap. From high temperature X-ray diffraction measurements, it was revealed that the miscibility gap between 0.5 and 0.9 of Pr2Ni1-xCuxO4+δ remained up to 700°C. Variation of the crystal structure of Ln2Ni1-xCuxO4+δ on kinds of Ln and Cu content could be explained by using tolerance factor calculated from ionic radius of Ln3+, Ni2+ and Cu2+.