Chlorophyll-fluorescence was measured by a handy PEA fluorometer in moss Grimmia pilifera P.Beauv in response to dehydration and rehydration following 8 day desiccation, and the result indicates that a threshold of relative water content (RWC) exists in moss Grimmia pilifera P.Beauv in terms of all the analyzed values of Fv/Fm、ETo/ABS、ETo/TRo、Area and RC/CSo during dehydration. A little change of chlorophyll-fluorescence was observed during dehydration above the RWC threshold, but a dramatic decrease occurred when the RWC became lower than the threshold. Upon rehydration after 8 day dehydration, the values of Fv/Fm、ETo/ABS、ETo/TRo、Area and RC/CSo recovered very rapidly, indicating that the photosynthetic apparatus of the moss Grimmia pilifera P.Beauv was not badly damaged by the dehydration in the condition, and the damage was reversible.