Abstract: Many studies were about the morphological development of Nostoc flagelliforme with the improvement of techniques of cultivation. Although the effects of the combination of temperature and illumination, nitrogen sources on development of N. flagelliforme have been described by many investigators, the effects of many environmental factors on developmental cycle of N. flagelliforme have not been studied. In the present investigation, the effects of different light intensity, nutrition and agar concentration on the morphological development of N. flagelliforme were studied. N. flagelliforme was collected at Siziwangqi, Inner Mongolia. The experimental strain was purified by our laboratory. The light intensity was designed with 0, N. flagelliforme was 20, 60 or 180 μmol/m2·s at 25℃. The development of N. flagelliforme was inhibited by complete dark and low illumination of N. flagelliforme was found to stay at sole filament and could not finish the developmental cycle. When N. flagelliforme was exposed to continuous illumination at 30 μmol/m2·s after the dark-grown for 16 days, synchronous development took place. The part of filaments split to form hormogonia and heterocyst. If these hormogone and heterocysts were continuously kept at illumination, the synchronous development would disappear. There was quick growth of N. flagelliforme at 10, 20, 30 and 60 μmol/m2·s, but its filament became yellow at 60 μmol/m2·s. The growth rate, carotenoid contents and polysaccharide contents of N. flagelliforme increased with the increase of illumination with liquid bubbled culture. In contrast to solid culture, the liquid bubbled culture of N. flagelliforme could tolerate high illumination. N. flagelliforme formed more heterocysts under low nutrient conditions. The location of heterocyst formation was varied in terminal, intercalary, contiguous place. Nitrogen-deficiency induced the formation of heterocysts. No heterocysts differentiation occurred in BG11 media. Phosphorous-deficiency induced the formation of more small cells, which was varied in shape and size. N. flagelliforme had the same development characteristic when agar concentration from 0.5% to 4%. It developed from hormogonia to filamentous aggregation then released filaments from aggregations. The filaments in aggregations were winding and twisting. The released hormogonia would repeat the above developmental cycle. The development of N. flagelliforme was not in-phase. In general, the above developmental morphology of N. flagelli forme coexisted. When agar concentration was at 0.5%—4%, the time of hormogonia released delayed with the increase of agar concentration. The release of hormogonia was investigated at 0.5%—1% agar concentration media for 8 days, and 2%—4% agar concentration media for 11 days. In solid culture with 1% agar concentration, stream of hormogonia migrating on the surface of agar was investigated. N. flagelliforme developed from hormogonia to filamentous aggregation when agar concentration was from 6% to 8%. Filaments were enveloped by thick sheath so that they were winding and could not be released. The results indicated that illumination and water content of culture medium were important for the development of N. flagelliforme.