This study examined the influence of cesium on seedling growth,its subcellular distribution,chemical forms and the damage mechanism adopted by Brassica juncea which cultured by Hoagland nutrient solution with varying doses of Cs⁺[ρ(Cs⁺)8.24(control),25,50,100 and 200 mg·L^-1].Determined the root,stem length by IMAGE-J;abstract subcellular components,and chemical forms of Cs⁺ by differential centrifugation,the chemical reagent extraction-test,then determined by atomic absorption spectroscopy.The results showed that:(1)Cs⁺ possesses strongly inhibitory effect on the growth of seedling.The root,and stem EC₅₀(concentration for 50% of maximal effect) were 112.09 mg·L^-1 and 118.42 mg·L^-1,respectively.(2)The accumulation of Cs⁺ in plant organs overall displayed the following sequence:leaf>root>stem.The subcellular components of Cs⁺ in Brassica juncea displayed the following sequence:soluble fraction>cell wall>organelles,which proportion was 52.86%-79.19%,20.81%-45.05% and 1.43%-9.00%,respectively.(3)The greatest amount of Cs⁺ was found in inorganic salt and the soluble,the two chemical forms relative contents in percentage were 88.02%-92.20%(in root),97.33%-100%(in stem) and 95.06%-100%(in leaf).Research shows that over dose of Cs⁺ showed toxic effects on Brassica juncea,such as inhibiting root and stem development,leading the leaf perished.The analysis indicated that Cs⁺ was mainly present in inorganic salt,and the soluble,and the two chemical forms have strong migration behavior and toxicology bio-availability and therefore can lead Cs⁺ transferred to the heavy metal toxicity sensitive area (such as organelles)more easily,causing cytoplasmic organelle functional damage,thus can lead plants show symptoms of poisoning obviously.