Nano-magnetic contrast agent has become one of the important research directions of MRI contrast agent; its unique physical and chemical properties and biological characteristics present a promising application prospect and play an increasingly important role in clinical and research work. Nano-magnetic contrast agent with special physicochemical and biological properties could be swallowed by human macrophages easily, causing changes of MRI signal intensity in lesion and possessing application prospect for quantitative diagnosis. This review discusses the recent progress on nano-magnetic contrast agent and their applications for imaging macrophages in human diseases.