In this study, the fatigue damage evaluation of seismic loadings was proposed and applied to an elbow of a three-dimensional simple piping system. The energy evaluation method was employed, focusing on energy balance of the total input energy and energy absorption of the piping system. Moreover, the concept of one-cycle momentary input energy was used for counting the loading cycle. Fatigue damage evaluation of the piping system was performed according to the maximum response of the elbow from the experimental results. The fatigue evaluation results were compared with the shaking test results of the piping system. Moreover, they were determined to be within the factor of 2 range, and were demonstrated to be applicable.