Abstract: The photosynthetic activities under salt stress of cyanobacterium Nostoc sphaeroides Kütz., an edible organism mainly located in some mountainous regions of China, were studied.The colonies were obtained in laboratory by cultivation.Harvested colonies (2 - 4mm in diameter) were treated with sodium chloride solution in order to reveal the effect of salt on the photosynthetic activities and the correlation among several photosynthetic parameters of the orgainism.The ratio of variable chlorophyll fluorescence (Fv) to maximum fluorescence (Fm) decreasing sharply when NaCl concentrations were over 0.2mol/L.The photosynthetic oxygen evolution rates (POERs) were negatively related with NaCl concentrations, especially when NaCl concentrations exceed 0.4mol/L, the rates decreasing more quickly with increasing NaCl concentrations, whereas the respiration rates were affected slightly by NaCl.The situations of PSI and PSII were similar to those of POERs.The result shows that changes of Fv/Fm values are coincided with POERs and PSII activities and there is relative low respiration rate in N.sphaeroides Kütz.