Formation of composites of dextran-coated Fe3O4 nanoparticles (NPs) and graphene oxide (Fe3O4-GO) and their application as T 2-weighted contrast agent for efficient cellular magnetic resonance imaging (MRI) are reported. Aminodextran (AMD) was first synthesized by coupling reaction of carboxymethyldextran with butanediamine, which was then chemically conjugated to meso-2,3-dimercaptosuccinnic acid-modified Fe3O4 NPs. Next, the AMD-coated Fe3O4 NPs were anchored onto GO sheets via formation of amide bond in the presence of 1-ethyl-3-(3-dimethyaminopropyl) carbodiimide (EDC). It is found that the Fe3O4-GO composites possess good physiological stability and low cytotoxicity. Prussian Blue staining analysis indicates that the Fe3O4-GO nanocomposites can be internalized efficiently by HeLa cells, depending on the concentration of the composites incubated with the cells. Furthermore, compared with the isolated Fe3O4 NPs, the Fe3O4-GO composites show significantly enhanced cellular MRI, being capable of detecting cells at the iron concentration of 5 μg mL–1 with cell density of 2 × 105 cells mL–1, and at the iron concentration of 20 μg mL–1 with cell density of 1000 cells mL–1.