The effects of exogenous spermidine(Spd) and spermine(Spm) on the leaves of Hydrocharis dubia under copper toxicity were investigated. The results showed that the content of putrescine (Put) increased and the contents of Spd and Spm decreased under copper stress in leaves, thereby the ratio of (Spd+Spm)/Put decreased significantly, which was concerned with the antioxidant ability in plants. Exogenous Spd or Spm markedly reversed these Cu-induced effects for partially enhanced the ratio of (Spd+Spm)/Put in leaves, which may be caused by decrease of polyamine oxidase (PAO) activity and increase of diamine oxidase (DAO) activity. It also markedly or highly significantly enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate specific peroxidase (APX) and glutathione reductase (GR), increased the contents of ascorbic acid (AsA), glutathione (GSH) and free proline (Pro), which was possibly the reason of decline of the genneration rate of superoxide anion (O?-2) and hydrogen peroxide (H2O2). So exogenous Spd and Spm restrained Cu-induced lipid peroxidation showed by decline of malondiadehyde (MDA) and soluble protein. Furthermore, exogenous Spd and Spm markedly reduced the accumulation of Cu and effectively maintained the balance of nutrient elements in Hydrocharis dubia leaves under Cu stress. These results suggested that exogenous Spd or Spm can enhance the tolerance of Hydrocharis dubia to copper by increasing the levels of endogenous Spd and Spm as well as the (Spd+Spm)/Put ratio.