Abstract: Heavy metals are common pollutants in aquatic ecosystems due to atmospheric sedimentation geological leakage, untreated industrial effluents. Compared to most other pollutants, they are not biodegradable and persistent in the environment for a very long period, Their accumulation in organism and bio magnification through food chain pose the great potential threat to aquatic organisms and to human health. So heavy metal pollution has become one of the most troublesome environmental problems and also received more and more attention. On the other hand, submerged macrophytes are directly exposed to heavy metals in water body and are more easily subjected to their toxicity In the present study, Potamogeton crispus, widespread in watin body in China, is cultivated in nutrition media containing different concentration of Cd2+ for 5d to investigate the toxic effect of Cd2+ on macrophtes, including:chlorophyll content, changes of protect enzyme activity(SOD, POD, CAT)and the ultrastructural damages to leaf cells, mainly to chloroplasts, mitochondria and nuclei. The results show that:SOD activity and chlorophyll content decrease nearly linearly with augmentation of Cd2+ concentration by 26. 44%and 60.55%, respectively, When the concentration was 50mg/L, statistical analysis indicate the changes of both indexes had reached significant levels( rSOD =-0.9766; rchl = -0.9132 ); Both POD and CAT activity reached the peak value at 1mg/L, 101.70% and 186.39% higher than that of the control, respectively. When treated with 20mg/L,cd2+ POD activity declined by 12.87% and 48.27% at 50mg/L as compared to the control Although the CAT activity began to decline when the treatment concentration rises further it still much higher than that of the control, by 86. 88% over the normal value even at 50mg/L within the exposing concentration range, It illustrated CAT has a quite strong tolerance to Cd2+ pollution. Among the three enzymes, SOD is the most sensitive, followed by POD and CAT. Transmission electron microscopy observation indicate that the higher the pollutant concentration is, the more serious the damages to the leaf cells is, such as swelling of chloroplasts at 1mg/L metal concentration, disruption and disappearance of chloroplast envelop at 10 and 20mg/L, and 50mg/L see the disintegration of chloroplasts;deforming and vacuolization of mitochondria at 1mg/L and 10mg/L concentration, respectively. The disintegration of nucleolus is found when treated with 10mg/L Cd2+, 50mg/L induced the disruption of nuclear membrane and vacuolization of nucleus. Disintegration of chloroplasts, vacuolization of mitochondria and vacuolization of nucleus are irreversible damage to organelles and their normal physiological functions are lost. The changes of leaf cells ultrastructure are consistent with the damage of their membrane systems, which may be caused by the disturbance of protect enzyme activity. It can be inferred that one main toxic mechanism of Cd2+ is to destroy the structural foundation that the normal metabolism necessarily need.