Abstract: Continuous depletion of the stratospheric ozone layer results in increasing solar ultraviolet radiation(UVR) reaching theearth’ s surface. Which has been considered to affect living organisms. Solar UVR may also affect a variety of physiological andbiochemical processes in the economic cyanobacterium, Spirulina platensis. However,little has been documented on this aspect.The present study evaluated the short2term effects of UVRon photochemical efficiencyof S. platensis in aerated and non2aeratedcultures while exposing it to full2spectrum solar radiation(PAB),solar radiation depleted of UVB(PA)and solar radiation deprivedof total UV(P:PAR).It was found that both UVR and PAR inhibited the photochemical efficiency during noontime(11:00213:00),however,UVR+PAR(PAB) resulted in higher inhibition. In aerated cultures, PAR resulted in 11 %～20 % compared to the initial value in themorning (07:00),UVR(PAB2P) brought about 9 %～31 % extra photoinhibition. While in non2aerated cultures,the photochemi2cal effciency could not be detected when exposed to UVR. The greatest inhibition was observed at noontime, and the recoverywas achieved in late afternoon(17:00) in both the aerated and non2aerated cultures. UVR increased the ratio of carotenoids orphycocyanin to chlorophyll a. Compared with PAB and P treatments, cells exposed to PA showed higher values of the pigmentratios,indicating a possibility that UVA may enhance the synthesis of carotenoids and phycocyanin. Although S. platensis wasnegatively affected by UVR, it tended to develop mechanisms counteracting the damaging effects of UVR during this short2termexposure period, such as down2regulation of the photosynthetic activity of accumulation of the carotenoids.