When a pantograph passes underneath a contact wire, the upward force causes a bending stress in the contact wire. If this stress is high and occurs frequently, there is a possibility that the contact wire will suffer a fatigue fracture. To estimate the fatigue life of a contact wire, we have been using the S-N curve prepared for a mean stress of 88.2 MPa. In addition, a contact wire is gradually worn down by running contact strips so that the mean stress governing its fatigue life gradually increases. To estimate the fatigue life of a worn contact wire correctly, therefore, the influence of the mean stress on contact wire fatigue needs to be examined. For this paper, we carried out fatigue tests on GT110 harddrawn copper contact wire at a mean stress of 154.2 MPa to obtain another S-N curve. Then, by using parameters that included the mean stress, we prepared a fatigue curve that integrates S-N curves for any mean stress. Finally, we estimated contact wire fatigue damage using this fatigue curve and proposed a new method to establish the allowable bending strain amplitude.