Abstract: The correlation between peroxidation damage of free radicals and ultrastructural changes of mesophyll cells under the condition of simulated Zn pollution was studied with Ottelia alismoides (L.) Pers. With the increace of Zn Stress,the content of chlorophyll,superoxide dismutase (SOD) activity and solub le protein content dropped significantly, peroxidase(POD) and catalase(CAT) activities rised at first and lowered later, and malonyl dialdehyde(MDA) content increased drastically. At low oncentration of Zn, the changes in ultrastructure were nuclei deformation, chloroplast swelling and disorder of thylakoid arrangement;serious damages in ultrastructure caused by greater Zn stress were indicated by scattered nucleoli,condensed chromatin,almost empty nuclei with nuclear membrane disrupted and nucleoplasm flowing into cytoplasm,swollen and partly dissolved cristae of mitochondria, disrupted and collapsed chloroplast envelopes,and some dissolved thylakoids that flew into cytoplasm.The results indicated that the changes of the ultrastructure of mesophyll cells were consistent with the damage of their membrane systems, which may be caused by the peroxidation damage of free radicals.