The low cycle fatigue behaviors of Ni-base superalloy KM 1557 with two different microstructures were investigated. The fatigue properties can be rationalized in terms of the Coffin-Manson eqaution relating plastic strain with the number of cycles to failure. For a low total strain amplitude(1.0%) the fatigue life of the Ni-base superalloy with α-W and γ′ is substantially less than that of other condition. Microstrucural examination revealed that microcracks initiated at the interface between matrix and α-W. This was attributed to the twin formation around α-W. During low cycle fatigue, the Ni-base superalloy with only α-W precipitates gives rise to a short period cyclic hardening followed by softening. However, the Ni-base superalloy with not only α-W but also γ′ gives rise to cyclic softening. The cyclic softening behavior of the Ni-base superalloy KM 1557 with two different microstructures was related to the strain localization due to γ′ shearing and the interface between α-W and matrix. This study mainly concerned with the role of interface between α-W and matrix, and twin formation.