Abstract：Anabaena sp. strain PCC7120 was a filamentous cyanobacterium that was capable of both oxygenic photosynthesis and aerobic nitrogen fixation. Heterocysts were developed from vegetative cells, which provided a suitable anaerobic environment for nitrogenase. The regulation of heterocyst development was influenced by several factors including concentrations, species of carbon and nitrogen sources, light intensity and NaCl concentration. In this paper, the effects of these factors on heterocyst differentiation in Anabaena sp. PCC7120 were studied. Results showed that NaNO3 and NH4Cl inhibited the development of heterocyst. But the elevated CO2 eliminated the inhibition of NaNO3 but not that of NH4Cl. The addition of NaHCO3 into the medium did not induce the production of heterocyst. Heterocyst frequency was enhanced by higher light intensity and NaCl concentration. It was found that heterocyst differentiation was induced in the presence of NaNO3 by elevated CO2, which was different from the opinion that Anabaena sp. strain PCC 7120 contained only vegetative cells in the presence of NaNO3.