OBJECTIVE: PEGylated superparamagnetic iron oxide (SPIO) is the most promising alternatives to gadolinium-based contrast agents (GBCAs) in MRI. This paper is to explore the imaging effects of PEGylated SPIO, which is influenced by particle sizes and surface polyethylene glycol (PEG) coating, using as MRI contrast agents at different magnetic field intensities. METHODS: Firstly, nine PEGylated monocrystalline SPIO nanoparticles with different nanocrystal sizes and different molecular weights PEG coating were prepared, and then physical and biological properties were analyzed. Finally, MRI imaging in vivo was performed to observe the imaging performance. RESULTS: Nine PEGylated monocrystalline SPIO nanoparticles have good relaxivities, serum stability, and biosecurity. At the same time, they show different imaging characteristics at different magnetic field intensities. Eight-nanometer SPIO@PEG5k is an effective contrast agent at 3.0 T ( / = 14.0), is an ideal - dual-mode contrast agent at 1.5 T ( / = 6.52), and is also an effective contrast agent at 0.5 T ( / = 2.49), while 4-nm SPIO@PEG5k is a - dual-mode contrast agent at 3.0 T ( / = 5.24), and is a useful contrast agent at 0.5 T ( / = 1.74) and 1.5 T ( / = 2.85). MRI studies in vivo at 3.0 T further confirm that 4-nm SPIO@PEG5k displays excellent - dual-mode contrast enhancement, whereas 8-nm SPIO@PEG5k only displays contrast enhancement. CONCLUSION: PEGylated SPIOs with different nanocrystal sizes and PEG coating can be used as , , or - dual-mode contrast agents to meet the clinical demands of MRI at specific magnetic fields.