The　water-soluble　l-cysteine-modified　CdSe/CdS　core/shell　nanocrystals　(expressed　as　CdSe/CdS/Cys　nanocrystals)　have　been　synthesized　in　aqueous　by　using　l-cysteine　as　stabilizer.　The　size,　shape,　component　and　spectral　property　of　CdSe/CdS/Cys　nanocrystals　were　characterized　by　high-resolution　transmission　electron　microscope　(HRTEM),　energy　dispersive　X-ray　fluorescence　(EDX),　infrared　spectrum　(IR)　and　photoluminescence　(PL).　The　results　showed　that　the　spherical　CdSe/CdS/Cys　nanocrystals　with　an　average　diameter　of　2.3　nm　have　favorable　fluorescent　property,　theirs　photostability　and　fluorescence　intensity　are　enhanced　greatly　after　overcoating　with　CdS.　The　cysteine　modified　on　the　surface　of　core/shell　CdSe/CdS　nanocrystals　renders　the　nanocrystals　water-soluble　and　biocompatible.　Based　on　the　fluorescence　quenching　of　the　nanocrystals　in　the　presence　of　calf　thymus　deoxyribonucleic　acid　(ct-DNA),　a　fluorescence　quenching　method　has　been　developed　for　the　determination　of　ct-DNA　by　using　the　nanocrystals　as　a　novel　fluorescence　probe.　The　pH　value　of　the　system　was　selected　at　pH　7.4,　with　excitation　and　emission　wavelength　at　380　and　522　nm,　respectively.　Under　the　optimal　conditions,　the　fluorescence　quenching　intensity　of　the　system　is　linear　with　the　concentration　of　ct-DNA　in　the　range　of　0.1-3.5　μg/mL　(r　=　0.9987).　The　detection　limit　is　0.06　μg/mL.　And　two　synthetic　samples　were　analyzed　satisfactorily.　©　2007　Elsevier　B.V.　All　rights　reserved.