Effects of cadmium concentrations(0, 0.01, 0.05, 0.1 and 0.5mg/L) on heterocyst frequency, nitrogenase and glutamine synthetase activities as well as ammonium nitrogen, free amino acid, soluble protein and total nitrogen contents in Azolla imbricata- Anabaena azollae symbiosis cultured in the growth medium were investigated under laboratory conditions. The results showed that the above mentioned parameters did not change significantly in A. imbricata-A. azollae symbiosis treated with 0.01mg/L Cd during 7 days treatment, indicating that A. imbricata-A. azollae symbiosis have a high tolerance to Cd. When Cd concentration was above 0.05mg/L, heterocyst frequency, the activities of nitrogenase and glutamine synthetase as well as the contents of soluble protein and total nitrogen decreased considerably with both increase in Cd concentration in the growth medium and duration of Cd treatment. At the same time, the ammonium content decreased in the early period of experiment, then increased rapidly, whereas free amino acid content increased gradually. These results suggested that higher cadmium treatment caused the disorder of nitrogen metabolism and reduced the accumulation of nitrogen in A. imbricata-A. azollae symbiosis.