Objective：To investigate the influence of particle size of superparamagnetic iron oxide（SPIO） nanoparticles on T1-T2 re－laxation efficiency，as well as the features of ultra-small SPIO（USPIO） as the T1-T2 double contrast enhancer in in vivo imaging. Methods：The coprecipitation method was used to prepare three types of SPIO nanoparticles with different particle sizes. The basic physical-chemical and magnetic properties of the samples were measured，and a 3.0 T MRI imager was used to investigate the imaging properties of USPIO as the T1-T2 double contrast agent in rats under different MRI sequences. Results：Three types of SPIO nanopar－ticles （A：14.14±4.94 nm；B：28.30±7.65 nm；C：93.81±33.01 nm） were successfully prepared and the relaxation properties of T1 and T2 at different magnetic field intensities were measured. SPIO nanoparticles with a smaller particle size had a lower r2/r1 ratio（0.5 T：2.14） and were the potential T1-T2 double contrast enhancers；such nanoparticles had a significantly lower r2/r1 ratio in low mag－netic field and might help with T1 contrast-enhanced imaging. The imaging of the rats showed that USPIO significantly reduced the T2 signal intensity of the liver on T2WI and increased the T1 signal intensity of the liver and the cardiovascular system on short-TE T1WI，with a longer time window for imaging than small-molecule contrast agents. Conclusion：Particle size is one of the important parameters affecting the relaxation efficiency of SPIO. USPIO has a lower r2/r1 ratio and can be used as the potential T1-T2 double contrast enhancer.