With the development of nanotechnology, nanocomposites have been used as bimodal contrast agents for magnetic resonance (MR) and computed tomography (CT) imaging. We have developed a facile method for the synthesis of iron oxide@bismuth sulfide magnetic core-shell nanocomposites. These bifunctional nanocomposites can be made water-soluble via PEG coating and present strong MRI/CT contrast enhancement. Evaluation of cytotoxicity by MTT assay shows that the nanocomposites have low cytotoxicity. The results illustrate that the nanocomposites have great potential as bimodal imaging agents for MR/CT.