Abstract: Cancer theranostics has attracted increasing attention in the area of nanooncology, where the therapeutic drugs and diagnostic imaging are integrated into a multifunctional nanoplatform. A theranostic nanoparticle can deliver therapeutic drugs and imaging agents simultaneously. Iron oxide based magnetic nanoparticles (MNPs) are one of the most typical theranostic nanoparticles and have many excellent properties, such as biosafety, superparamagnetism, and tunable surface modifications and functionalizations. Moreover, they have drug loading capacity along with the distinctive properties of T2 weighted magnetic resonance imaging (T2W MRI), magnetic targeting, and magnetic hyperthermia. Presently, iron oxide based MNPs are being widely used in cancer theranostic research. This paper introduces the general structure of iron oxide based MNPs and reviews their applications in cancer dual/multiple modal imaging (T2W MRI combining T1W MRI, CT, optical imaging, PET/SPECT, and ultrasound) and therapy (chemotherapy, photodynamic therapy, photothermal therapy, and magnetic hyperthermia).