The objective of the present study was to gain better insight into the physiological mechanisms on the effects of copper (Cu) on photosynthesis and active oxygen metabolism in three-colored amaranth plant (Amaranthus tricolor). Three- colored amaranth seedlings were subjected to different Cu levels in soils during the entire experimental period. The parameters of growth, photosynthesis, mineral elements contents, and active oxygen metabolism were investigated using plant physiological methods. The results showed that 2.0 and 4.0 mmol Cu kg-1 treatments decreased the whole plant biomass to 91 and 73% of the control, respectively. The net photosynthetic rate (Pn) and the stomatal conductance (gs) were similarly reduced in the third leaves of three-colored amaranth seedlings treated with 2.0 and 4.0 mmol Cu kg-1 soil, respectively. None of the investigated Cu levels decreased the internal CO2 concentration (Ci). The effect of Cu on the potential efficiency of photosystem Ⅱ(Fv/Fm) was negligible, whereas the effect of Cu on the PS Ⅱ quantum efficiency (ΦPSⅡ) after plant adaptation in actinic irradiation was more noticeable. On the other hand, decreases in water percentage, contents of photosynthetic pigments and mineral elements including Fe, K, and Mg, and significant increase in the Cu content were observed in the third leaves of Cu-treated plants. Superoxide dismutase (SOD) and peroxidase (POD) activities as well as the proline (Pro) content significantly increased in the third leaves of the three-colored amaranth seedlings treated with 2.0 and 4.0 mmol Cu kg-1 soil, while catalase (CAT) and ascorbate peroxidase (APX) activities as well as the contents of carotenoid (Car), glutathione (GSH), and ascorbic acid (AsA) decreased, and accompanied by the increases in the contents of hydrogen peroxide (H2O2), superoxide anion (O2·- ), and malondialdehyde (MDA), and electrolyte leakage. As a result of the imbalance of active oxygen metabolism, Pn and ΦPSⅡ decreased, and peroxidization enhanced under levels of 2.0 and 4.0 mmol Cu kg-1 soil. Finally, the growth of three-colored amaranth plant was significantly inhibited. The objective of the present study was to gain better insight into the physiological mechanisms on the effects of copper (Cu) on photosynthesis and active oxygen metabolism in three-colored amaranth plant (Amaranthus tricolor). Three-colored amaranth seedlings were subjected to different Cu The parameters of growth, photosynthesis, mineral elements contents, and active oxygen metabolism were investigated using plant physiological methods. The results showed that 2.0 and 4.0 mmol Cu kg-1 treatments decreased the whole plant biomass to 91 and 73% of the control, respectively. The net photosynthetic rate (Pn) and the stomatal conductance (gs) were respectively reduced in the third leaves of three-colored amaranth seedlings treated with 2.0 and 4.0 mmol Cu kg-1 soil, respectively . The effect of Cu on the potential efficiency of photosystem II (Fv / Fm) was ne gligible, whereas the effect of Cu on the PS II quantum efficiency (ΦPSII) after plant adaptation in actinic irradiation was more noticeable. On the other hand, decreases in water percentage, contents of photosynthetic pigments and mineral elements including Fe, K, and Mg , and significant increase in the Cu content were observed in the third leaves of Cu-treated plants. Superoxide dismutase (SOD) and peroxidase (POD) activities as well as the proline (Pro) content significantly increased in the third leaves of the three- colored amaranth seedlings treated with 2.0 and 4.0 mmol Cu kg-1 soil, while catalase (CAT) and ascorbate peroxidase (APX) activities as well as the contents of carotenoid (Car), glutathione (GSH), and ascorbic acid (AsA) , and accompanied by the increase in the contents of hydrogen peroxide (H2O2), superoxide anion (O2 · -), and malondialdehyde (MDA), and electrolyte leakage. As a result of the imbalance of active oxygen metabolism, Pn and ΦPSⅡ decrease d, and peroFinally, the growth of three-colored amaranth plant was significantly inhibited.