Abstract: In this article, the microstructure, micro-hardness and low-cycle fatigue property of a 12Cr/30Cr2Ni4MoV welded joint have been studied. The results show that the microstructure of the 12Cr is lath tempered martensite, 30Cr2Ni4MoV is lath tempered martensite and granular tempered bainite. The microstructure of the buttering layer (BL) and weld metal (WM) is granular tempered bainite. WM is composed of columnar grain regions and equiaxed grain regions alternately. 30Cr2Ni4MoV has higher micro-hardness than 12Cr, and the BL is the lowest. Micro-hardness in the heat-affected zone (HAZ) is gradient change. 12Cr, 30Cr2Ni4MoV and BL show cyclic softening characteristics, while WM has initial cycle hardening and subsequent cyclic softening characteristics. BL has the highest fatigue transition life; the 12Cr/BL interface is the weakest link of the low-cycle fatigue of the entire heterogeneous welded joint. The second phase particles distributed in the grain boundary or sub-grain boundary of the columnar grains have uncoordinated deformation with the matrix of WM, which result in the initiation of fatigue cracks in second phase