The toxic effect of various concentrations of Hg²⁺ on the photosythentic O₂ evolution respiration rate the permeability of cell membrane nitrate reductase activity and the ultrastructure changes in leaf cells of Hydrocharis dubia were studied. The results showed that the photosythentic O₂ evolution decreased respiration rate and nitrate reductase activity first increased and then decreased while the permeability of cell membrane was increased. Lltrastructure observation revealed that the chromatin condensed the thylakoids of chloroplast and cristae of mitochondria swelled at the early stage when the leaves suffered the poison of Hg²⁺. With the increasing of the poison nuclear membrane disrupted the chromatin and nucleoplasm disappeared, chloroplast disintegrated and mitochondria vesiculated.