Cadmium　(Cd)　pollution　of　the　soil　is　an　important　global　environmental　issue　owing　to　its　great　toxicity.　The　study　of　metal　isotope　fractionation　is　a　novel　technique　that　could　be　used　to　identify　and　quantify　metal　uptake　and　transport　mechanisms　in　plant.　In　this　study,　cadmium　tolerant　Ricinus　communis　and　hyperaccumulator　Solanum　nigrum　have　been　cultured　in　different　Cd　concentration　nutrient　solutions.　The　Cd　isotope　values,　metal　elements　concentrations　in　the　organs　(root,　stem　and　leaf)　in　the　two　plant　species　have　been　measured　during　the　growth　periods　(10d,　15d,　20d,　25d,　and　30d).　The　results　indicate　that　the　organs　of　S.　nignun　could　be　enriched　with　lighter　Cd　isotopes　compared　with　R.　communis.　In　addition,　the　Cd　isotope　fractionation　become　smaller　when　the　plants　were　subjected　to　high　Cd　toxicity,　which　indicates　that　Cd　isotope　fractionation　reflected　the　extent　of　Cd　toxicity　to　plants.　This　study　advances　our　current　view　of　Cd　translocation　machination　in　plants.