Two strains of Aphanizomenon flos-aquae were isolated from spring cyanobacterial blooms in Haigeng Bay which located in the northern part of Lake Dianchi. Both the growth characteristics and heterocyst formation as well as nitrogen fixation ability of A. flos-aquae were investigated under different low concentrations of sodium nitrate, and in nitrogen-free culture medium, respectively. The results showed that A. flos-aquae strains were able to grow in the BG-11 medium with various low concentrations of nitrate, and the biomass reached a very high level. The biomass increased with the nitrate-nitrogen concentration increasing, but there was no significant difference when nitrate-nitrogen concentration was above 0.5 mg/L. Induction of heterocyst differentiation was rapid when A. flos-aquae was cultured in N-deficient BG-11 medium, and after reaching a maximum of 72% approximately, the filament with heterocyst began to swing lower. The total nitrogen in nitrogen-free medium increased gradually as nitrogen fixation proceed, and nitrogen concentration was close to 30 mg/L at day 43.