Multifunctional nanoparticles combining diagnostic and therapeutic agents into a single platform make cancer theranostics possible and have attracted wide interests in the field. In this study, a multifunctional nanocomposite based on dextran and superparamagnetic iron oxide nanoparticles(SPIO) was prepared for drug delivery and magnetic resonance imaging(MRI). Amphiphilic dextran was synthesized by grafting stearyl acid onto the carbohydrate backbone, and micelle was formed by the resulted amphiphilic dextran with low critical micelle concentration at 1.8 mg L-1. Doxorubicin(DOX) and a cluster of the manganese-doped iron oxide nanoparticles(Mn-SPIO)nanocrystals were then coencapsulated successfully inside the core of dextran micelles, resulting in nanocomposites with diameter at about 100 nm. Cell culture experiments demonstrated the potential of these Mn-SPIO/DOX nanocomposites as an effective multifunctional nanoplatform for the delivery of anticancer drug DOX with a loading content(DLC) of 16 %. Confocal laser scanning microscopyreveals that the Mn-SPIO/DOX had excellent internalization ability against MCF-7/Adr cells after 2-h labeling compared with free DOXáHCl. Under a 3.0-T MRI scanner, Mn-SPIO/DOX nanocomposite-labeled cells in gelatin phantom show much darker images than the control. Their transverse relaxation(T2) rate is also significantly higher than that of the control cells(33.9 versus 2.3 s-1). Our result offers an effective strategy to treat MCF-7/Adr at optimized low dosages with imaging capability. Multifunctional nanoparticles combining diagnostic and therapeutic agents into a single platform make cancer theranostics possible and have attracted wide interests in the field. In this study, a multifunctional nanocomposite based on dextran and superparamagnetic iron oxide nanoparticles (SPIO) was prepared for drug delivery and magnetic resonance imaging (MRI). Amphiphilic dextran was synthesized by grafting stearyl acid onto the carbohydrate backbone, and micelle was formed by the resulting amphiphilic dextran with low critical micelle concentration at 1.8 mg L-1. Doxorubicin (DOX) and a cluster of the manganese- doped iron oxide nanoparticles (Mn-SPIO) nanocrystals were then coencapsulated successfully inside the core of dextran micelles, resulting in nanocomposites with diameter at about 100 nm. Cell culture experiments demonstrated the potential of these Mn-SPIO / DOX nanocomposites as an effective multifunctional nanoplatform for the delivery of anticancer drug DOX with a loading content (DLC) of 16%. Confocal laser scanning microscopyreveals that the Mn-SPIO / DOX had excellent internalization ability against MCF-7 / Adr cells after 2-h labeling compared with free DOXα HCl. Under a 3.0-T MRI scanner, Their transverse relaxation (T2) rate is also significantly higher than that of the control cells (33.9 versus 2.3 s-1). Our result offers an effective strategy to treat MCF-7 / Adr at optimized low dosages with imaging capability.