The anode purge is a crucial process for the fuel cell long time operation because when the hydrogen is supplied in a circulation mode, any impurities present in hydrogen will gradually accumulate which lead to output voltage loss. A mathematical model is proposed for the purge process based on some operational purge parameters. The governing equations are solved and the effect of purge process on the stack working parameters is analyzed. Purge operational parameters are determined in such a way that the minimum pressure fluctuations in the anode compartment and a compromise between the minimum voltage loss and minimum hydrogen waste are achieved. A semi-stable condition is introduced and indicated that the behavior of voltage loss and hydrogen waste at this condition with respect to purge stop time (duration which the purge valve is closed) is semi-logarithmic. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.