NiFe(2)O(4)presents high redox activity and good promoted application prospect in chemical looping processes. However, phase segregation caused by outward diffusion of Fe cations often lead to low cycling stability for NiFe2O4. In this study, the inherent mechanism of phase segregation for NiFe(2)O(4)was investigated deeply. The results indicated that reduction degree exhibited significant influence on the phase segregation of NiFe(2)O(4)in successive redox cycles. When NiFe(2)O(4)was reduced to Ni and Fe(3)O(4)in redox cycles, NiFe(2)O(4)displayed stable redox activity without phase segregation. As the reduction degree reached to FeO and FeNi, a needle-like structure with Fe enrichment was formed on the surface of the first cycled NiFe2O4. In the subsequent redox cycles with deep reduction degree, both severe phase segregation and deactivation were occurred for spent NiFe(2)O(4)sample. It can be seen that phase segregation is the main reason for the deactivation of NiFe(2)O(4)instead of surface sintering in redox cycles. The results of this work provide guidance for the development of NiFe2O4-based oxygen carriers with high redox performance.