Superparamagnetic　nanoparticles　functionalized　with　carboxymethyl　dextran　(CM-dextran)　were　synthesized　by　a　two-step　method.　First,　the　magnetic　nanoparticles　(MNPs)　coated　with　dextran　(M-w　approximate　to　20000)　were　prepared　by　co-precipitation　of　Fe2+　and　Fe3+　ions.　Then,　dextran　on　the　surface　of　MNPs　reacted　with　monochloroacetic　acid　(MCA)　in　alkaline　condition.　The　influences　of　temperature　and　reactant　concentration　on　the　amount　of　-COOH　on　the　surface　of　nanoparticles　were　systematically　studied.　The　obtained　MNPs　coated　with　CM-dextran　were　stable　over　the　entire　range　of　pH　and　NaCl　concentration.　The　MRI　experiment　indicated　that　the　CM-dextran　MNPs　could　potentially　be　used　as　MRI　contrast　agents　for　magnetic　resonance　molecular　imaging.　(C)　2011　Elsevier　B.　V.　All　rights　reserved.