Structural phase diagram of Nd2Ni1-xCuxO4+δ, a promising new cathode material for solid oxide fuel cells, was established. The single phase with K2NiF4-type crystal structure was obtained for the specimens with x≤0.3. Single phase with crystal structure of T'-phase was obtained for Nd2CuO4. For the specimens with x between 0.4 and 0.9, phase separation into K2NiF4 structure and T'-phase structure due to miscibility gap was observed. In the miscibility gap, Cu content, x, of both K2NiF4 phase and T'-phase was constant and the compositional ratio of T'-phase linearly increased with increasing x. High temperature X-ray diffraction measurements revealed that the miscibility gap remained up to 1000°C for the specimens with x between 0.4 and 0.9.