Abstract: In order to study the toxicity mechanisms of naphthalene (NAP) on Ceratophyllum demersum, 1/10Hoagland nutrient solution hydroponics was used in this study. The contents of chlorophyll, soluble protein, glutathione (GSH ) and malondialdehyde (MDA), and the activities of superoxide dismutase (SOD) and peroxidase (POD) in C. demersum were investigated under the treatments of six NAP concentrations (1.25,2.5,5, 10,0and 40mg/L). Results showed that there was no difference on all measured characteristics for C. demersum with the concentrations of 1.25and 2.5mg/L NAP (P>0.05). However, the physiological activity of C. demersum was affected significantly (P<0.05) as the concentration of the NAP reached 40mg/L. At the late stage (the fifth and seventh days) of NAP stress, as NAP concentration increased, the contents of chlorophyll, soluble protein and the activity of SOD decreased, while GSH and MDA increased which indicated the increases of lipid peroxidatio. Thus, the contents of chlorophyll, MDA, and the activities of SOD could be characterized the toxicity response of C. demersum to NAP.