Abstract: Extracellular polysaccharides (EPS) play an essential role in adhesion for crust cyanobacteria to form the biological soil crust. The mechanisms of how the normally ball-shape filamentous cyanobacteria Nostoc sp. cements sand grains remains to be elucidated. To understand the mechanisms we investigated the effects of light intensity and nitrogen source on the the secretion of EPS including releleased exopolysaccharides (RPS) and capsular polysaccharides (CPS) in the subtype of Nostoc sp. that dominates the formation of the biological soil crust. We tested two forms of nitrogen source-N2 and nitrate, and the light intensities at 40 and 80 E/(m2s). The results showed that the biomass of Nostoc sp. in diazotrophic condition was almost the same to that in non-diazotrophic condition. The production of RPS, CPS and EPS in Nostoc sp. was elevated along with the increase in light intensity in nitrate-grown cultures. However, light intensity did not affect the yields of RPS, CPS and EPS in N2-fixing cultures. Studies in Nostoc sp. metabolism showed that the intracellular contents of soluble sugar and sucrose were significantly higher in the presence of nitrate compared to N2. Interestingly a positive correlation between the sucrose content and RPS/CPS production was only observed at the light intensity of 80 E/m2s, whereas in the N2-fixing cultures, the total contents of intracellular carbohydrate were negatively correlated with CPS production at both tested light intensities. These results suggested that Nostoc sp. had the advantages of utilizing nitrogen and adapting to the light intensity. Although Nostoc sp. in the logarithmic phase displays a rapid growth rate, they are able to secrete a considerable amount of EPS which enables them to cement sand grains before forming the spherical hormogonium. Thus, these results further suggested that, during the early stage of the growth of Nostoc sp. in the poor soil surface, the secreted extracellular EPS may originate from the intracellular products of fixing carbon, and sucrose was probably the source for the EPS biosynthesis under high intensity of light.