Magnetite (Fe3O4) nanoparticles were synthesized by thermal decomposition method of iron (III) acetylacetonate (Fe(acac)3) in organic solvent, then, was performed phase transfer and coated with poly acrylic acid (PAA). The crystalline structure, morphology and magnetic properties of samples were characterized by X-ray diffraction patterns (XRD), transmission electron microscope (TEM), fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and vibrating sample magnetometer (VSM). The PAA-cappped Fe3O4 nanoparticles has spinel single phase structure with average size of 10 nm and high saturation magnetization (up to 64 emu/g). The amounts of Fe3O4 magnetic particles in the samples were 84.27% weight for the sample Fe3O4@PAA. The magnetic Fe3O4 nano material was formed in liquid phase with high homogeneity, mono-dispersion, as well as good stability promised a potential application in biomedical