Crack nucleation along deformation twin boundary has been investigated in an Fe-20%Cr alloy. The deformation twins were introduced by compression test in liquid nitrogen. Push-pull fatigue tests on the twinned specimens were carried out at room temperature. Two kinds of cracks were detected in the vicinity of the twin boundaries: one was a serrated intragranular crack and the other was an intergranular crack formed along the twin boundary. From a monotonic tensile test on a twinned specimen, it is confirmed that the serrated crack was generated as a result of twin boundary migration during detwinning process, regardless of fatigue cycling. After the fatigue loading was sufficiently repeated, the twin boundary migration and the related serrated crack nucleation were no longer observed. Instead, extrusions were generated on the twin boundaries. The fatigue cracks were finally nucleated along the extrusions. When the twinned specimen was annealed, fatigue property was improved. This is because the nucleated twin-boundary cracks were arrested at the edge of twin bands which were highly fragmented by the annealing.