In this research,Sesuvium portulacastrum,a mangrove plant was used as material.Watercultured seedlings of samphire,which are at the same growth status,were treated in nutrient solution with different concentrations of Hg²⁺ in it.The ultrastructure change of S.portulacastrum mesophyll cell,which was respond to different concentrations of Hg²⁺,was observed by TEM(Transmission Electron Microscope) to probe the effect of Hg²⁺ to S.portulacastrum mesophyll cells.The result shows that:Hg²⁺ can lead to different poisoning symptoms to chloroplast,mitochondria,nucleus and membrane system in diachyma cells.With the increase of Hg²⁺ concentration,the number of chloroplast in diachyma cells is decreasing.The shapes of chloroplast change from ship type to long or giant type.The thylakoid system is also injured,and grana lamella is getting dim.The number of mitochondria is determined by Hg²⁺ concentration,and its shape changes from rodlike to round or ellipse.The double membrane system and cristae of mitochondria is also becoming dim.The nuclei are injured in different degrees and the nucleolus changes from one to more and finally disappear.The membrane forms protuberances into the intracytoplasmic and then change to bubbles.The cell will disintegrate with the decrease of organelles under the stress of high concentration of Hg²⁺.