The mechanism by which Zn promotes Cu toxicity in duckweed Spirodela polyrhiza L. was investigated in order to understand the possible interaction between these two metals. Cu uptake was gradually declined by Zn. The induction of oxidative stress is shown by increased levels of lipid peroxidation, total peroxide, superoxide anion and lipoxygenase activity. Zn interaction reduced the oxidative damage. However, only Zn-treated plants did not show alteration in the above observed parameters. The activities of antioxidant enzymes catalase, ascorbate peroxidase and peroxidase showed a very high increase in activity in Cu+Zn treatment as compared to Cu or Zn alone-treated plants. Thus, this study demonstrates that zinc reversed the effect of copper, combating against Cu induced oxidative damage and improvement of duckweed's growth and toxicity under natural condition.