Well-dispersed Fe3O4 nanoparticles were synthesized using chemical coprecipitation and the addition of organic acid as adherent in an aqueous solution. The resulting iron oxide particles were manipulated as small as 6 nm diameter in size. The nanoparticles exhibit cationic surface mostly decorated with terminal -NH3+. The in vitro cytotoxicity test revealed excellent biocompatibility to further explore for their potential in vivo applications. NMR measurements showed significantly reduced water proton relaxation times T1 and T2. The MR images of the nanoparticles in water and serum were then investigated using 1.5T clinical MR imager.