Superparamagnetic iron oxide (SPIO) nanoparticles were synthesized by thermal decomposition of the iron(III) acetylacetonate (Fe(acac)₃) in poly(ethylene glycol) (PEG) containing poly(ethylene imine) (PEI) as modifying agents. The samples were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), superconductivity quantum interference device (SQUID), thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FT-IR), particles & zeta potential analyser, and small animal 7 T MR scanner. The results show that approximate equiaxial shaped nanoparticles have been obtained by the method, the size of nanoparticles increases from (5.2±1.0) nm to (10±1.7) nm and their crystallinity increases with increasing the synthesized temperature from 180℃ to 260℃, and the magnetite nanoparticles show superparamagnetic behavior at room temperature with high saturation magnetization. The surfaces of SPIO were modified by PEG and PEI which provided excellent water dispersibility and stability for a long time (more than 90 d). The T₁ and T₂ weighted MR images of 10 nm-sized SPIO synthesized at 260℃ showed good contrasting effect, exhibiting a low r₁ relaxivity of 1.65 mmolL^-1S^-1 and a high r₂ relaxivity of 142.99 mmolL^-1S^-1with a high r₂/r₁ ratio of 86.6. Magnetic resonance imaging in vivo demonstrated that the SPIO could be excellent T₂ contrast agents.