Of the fuel cells being studied, the proton exchange membrane fuel cell (PEMFC) is viewed as the most promising for transportation. Yet until today, the commercialization of the PEMFC has not been widespread in spite of its large expectation. Poor long term performances or durability, and high production and maintenance costs account for the main reasons. For the final commercialization of fuel cell in transportation field, the durability issue must be addressed, while the costs should be further brought down. In the meantime, health-monitoring and prognosis techniques are of great significance in ensuring the normal operation of the fuel cell and preventing or predicting its likely abrupt and catastrophic failure. In this paper, an analytical formulation of a damage accumulation law for fuel cell is presented. To identify the damage variables that have the closest correlations to the aging process and to derive the aging dynamics, experiment was conducted at Oak Ridge National Laboratory's Fuel Cell and Diagnostics Lab. Two home-made Nafion 117 proton exchange membrane fuel cells, one with graphite bipolar plates and one with nitrided Fe-20Cr-4V alloy foils have been running under a long time cycling test in two commercially available fuel cell test stands. Experiment data obtained from the durability tests are curve fitted to derive the aging equation, whose equivalence to the Palmgren-Miner fatigue model used for mechanical components is proved. The proposed prognostic algorithm can be used to predict the remaining life of the fuel cell.