The toxicity and effects of diethyl phthalate (DEP), a potent allelochemical, on the growth of greater duckweed were studied. Biochemical analyses and physiological methods were combined to investigate oxidative stress, adverse effects and their mechanisms in greater duckweeds grown in 0-2 mM of diethyl phthalate (DEP) after cultivation for 7 days. The results showed that J-shaped concentration response curves were displayed in hydrogen peroxide (H2O2), ascorbic acid (ASA) and dehydroascorbate (DHA) levels, and ascorbate peroxidase (APX) and gualacol peroxidase (POD) activities, indicating reduced oxidative stress and toxic effect. The inverted U-shaped curves were exhibited in relative growth rate (RGR), fresh weight/dry weight (FW/DW) ratio, total chlorophyll content, total soluble thiols, and glutathione reductase (GR) activity, revealing beneficial effect in plant growth. The inverted U-shaped curves were also found in malondialdehyde (MAD) and superoxide radical (O2-) contents with the increasing concentration of DEP, indicative of enhanced oxidative stress. The results suggest that DEP is toxic to the greater duckweed by inducing oxidative stress and antioxidative enzymes may play important roles in the defense strategy against DEP toxicity.