Superparamagnetic iron oxide nanoparticles (SPIONs) offer unique properties for magnetic resonance imaging (MRI). Targeting imaging of rheumatoid arthritis in vivo requires a specific, magnetic sensitive and ultra-stable MRI contrast agent. In this study, SPIONs with a preferable colloid stability and optimized size were obtained by using an in situ polyol method with the diblock copolymer PEG-b-PAA acting as a surface ligand. Increasing the degree of polymerization (DP) of PAA from 18 to 36 to 57 led to the decreasing size of the iron oxide nanoparticles from 52 nm to 17 nm to 9 nm, respectively. Folic acid was conjugated onto PEG-PAAx@SPION as a specific targeting molecule for activated macrophages in a rheumatoid arthritis joint. To evaluate the stability and magnetic properties of the particle, a series of tests were conducted to evaluate and optimize the nanoparticles. In vitro endocytosis experiments confirmed the better performance of the folic acid conjugated SPIONs than the non-folic acid modified SPIONs. In vivo MRI clearly demonstrated the significant signal diminishment of the arthritis joint in antigen induced arthritis (AIA) rats by intravenous injection of the optimized nanoparticles FA-PEG-b-PAA36@SPION. These results indicated that FA-PEG-b-PAA36@SPION could serve as a promising candidate for the MRI of rheumatoid arthritis.