Abstract: In recent years, continuing depletion of the stratospheric ozone layer by atmospheric pollutants, in particular chlorofluorocarbons ( CFCs), has resulted in an increasing incidence of solar UV-B( 280-320nm)at the Earth' s surface. The process of ozone depletion is expected to increase and spread to abroader range of altitudes and latitudes throughout most of the current century. The effect of depletion of the stratospheric ozone layer has been taken place not only in the polarregions, but also in temperate regions. Besides exerting a variety of adverse effects on terrestrial vegetation, enhanced UV-B radiation is also known to penetrate fairly deep in freshwater and marine environments,in which it affects both the plankton in the euphotic zone as well as benthic organisms. Enhanced UV-B radiation has been considered as an important global environmental problem and resulted in important effects to mankind and the entire global ecosystem. Therefore, many investigations have been carried out to elucidate the resulting biological effects. The research results showed that the dramatic decreaserin development, biomass, photosynthesis and respiration of plants under enhanced UV-B radiation. Cyanobacteria are ecologically and economically important group living prokaryote of single cell in the world, which has important ecological site, characteristic ability of metabolism and important value of research. Cyanobacteria have a cosmopolitan distribution ranging from hot springs to the Arcticand Antarctic regions. Being photosynthetic prokaryotes, they formed an important component of the nitrogen-fixing microbial population both in aquatic and terrestrial ecosystems especially in tropical rice fields. Cyanobacterium also has significant economic potentials, being used in the production of food for humans and animals, fertilizers and cosmetics, to name only a few. In this paper, the response of a kind of cyanobacterium Spirulina platensis 794 to enhanced UV-B stress under laboratory condition were studied by the determination of some physiological indexes. It was shown that the biomass decrease, content of chlorophyll a and carotinoid in cell of Spirulina platensis 794 reduce under UV-B stress, which resulted in certain inhibition on growth and development of Spirulina platensis. But the content of cell plasmic protein increased, it maybe was an adaptive response of Spirulina platensis to stress. Under enhanced UV-B stress, not only the content of MDA but also O2 increased in Spirulina platensis cells. It suggested that the decrease of content of chlorophyll a and the increase of content of MDA in cell related with the oxidation harm of O2.