Uniform, highly water-dispersible and ultra-small Fe3O4 nanoparticles were synthesized via a modified one-step coprecipitation approach. The prepared Fe3O4 nanoparticles not only show good magnetic properties, long-term stability in a biological environment, but also exhibit good biocompatibility in cell viability and hemolysis assay. Due to the ultra-small sized and highly water-dispersibility, they exhibit excellent relaxivity properties, the 1.7 nm sized Fe3O4 nanoparticles reveal a low r(2)/r(1) ratio of 2.03 (r(1) = 8.20 mM(-1) s(-1), r(2) = 16.67 mM(-1) s(-1)); and the 2.2 nm sized Fe3O4 nanoparticles also appear to have a low r2/r1 ratio of 4.65 (r(1) = 6.15 mM(-1) s(-1), r(2) = 28.62 mM(-1) s(-1)). This demonstrates that the proposed ultra-small Fe3O4 nanoparticles have great potential as a new type of T-1 magnetic resonance imaging contrast agents. Especially, the 2.2 nm sized Fe3O4 nanoparticles, have a competitive r(1) value and r(2) value compared to commercial contrasting agents such as Gd-DTPA (r(1) = 4.8 mM(-1) s(-1)), and SHU-555C (r(2) = 69 mM(-1) s(-1)). In vitro and in vivo imaging experiments, show that the 2.2 nm sized Fe3O4 nanoparticles exhibit great contrast enhancement, long-term circulation, and low toxicity, which enable these ultrasmall sized Fe3O4 nanoparticles to be promising as T-1 and T-2 dual contrast agents in clinical settings.