Abstract: We investigated the effects of nitrophenol stress on the growth, antioxidant properties, and PSⅡ photosynthetic characteristics of Oryza sativa L. seedlings, as well as the alleviative effects of exogenous melatonin on nitrophenol stress, through liquid culture experiments. Results showed that with the increase in nitrophenol concentration, the plant height, root length, below-ground part dry weight, above-ground part dry weight, plant dry weight, effective photochemical quantum yield of PSⅡ(Y(Ⅱ)), photochemical quenching coefficient(qP), non-cyclic electron-transport through PSⅡ(ETR), and chlorophyll content of leaves all decreased. Conversely, the non-photochemical quenching coefficient(qN, NPQ) of the leaves increased with increasing nitrophenol concentration. Simultaneously, the accumulation of reactive oxygen species(ROS)(including hydrogen peroxide and superoxide anion radicals(H₂O₂ and O₂^·-)), activity of antioxidant enzymes(including superoxide dismutase(SOD), peroxidase(POD), catalase(CAT), and ascorbate peroxidase(APX)), and content of the osmotic regulator(soluble protein and sugar) of roots first increased then decreased with increasing nitrophenol concentration. Without nitrophenol stress, the below-ground part dry weight, soluble sugar content, and SOD activity of the roots, PSⅡ photochemical efficiency, and chlorophyll content of the leaves all significantly increased under exogenous melatonin treatment compared with the control. In comparison with nitrophenol treatment, the combination of nitrophenol and melatonin treatment significantly alleviated the inhibition of nitrophenol stress on seedling growth, PSⅡphotochemical efficiency, and chlorophyll synthesis, and reduced ROS levels, antioxidant enzyme activities, and osmotic regulator content in roots. Thus, the addition of exogenous melatonin can significantly alleviate the adverse effects of nitrophenol stress on O. sativa L. seedlings.