Abstract: In order to clarify the physiological and biochemical basis of Nostoc commune lived in different environmental light levels,their daily changes of F_v/F_m,photosynthetic characteristics and content of carotenoids and chlorophyll were studied.Compared with shade-acclimated N.commune,sun-acclimated N.commune had higher light saturation point,higher content of carotenoids and higher ratios of carotnenoids to chlorophyll as while as lower ascending slope of light-limited part of P-I curve,lower dark respiration,lower F_v/F_m and lower content of chl a and MAAs on the basis of dry weight or surface areas.And their maximum photosynthetic rate and light compensation point were insignificantly different.Both of them had no obvious photorespiration.Shade-acclimated N.commune was more sensitive to high light than sun-acclimated ones.If the sunlight was high enough after rain,they both were photoinhibited,but they could recover immediately at lower light levels or at night.Results showed that photosynthetic characteristics and pigmentation for sun-acclimated and shade-acclimated N.commune were significantly different and these might be responsible for their adaptations to different light levels.