Nano-FexOy (including γ-Fe2O3, α-Fe2O3 and Fe3O4) particles were prepared by acetylacetonate iron [Fe(C5H7O2)3] thermal decomposition under air condition at low temperature. X-ray diffusion (XRD), transmission electron microscopy (TEM), electron diffraction pattern (EDP), and specific surface area were carried out to characterized these nanoparticles. The influencing factors of experiment were discussed and studied, and the results show that γ-Fe2O3, α-Fe2O3 and Fe3O4 in different morphology can be obtained respectively by controlling the experiment conditions. The primary particle of Fe3O4 is about 16 nm; the primary particle of γ- Fe2O3 is about 20 nm and arranges compact. The second nanoparticle of γ- Fe2O3 presents catenarian and has the tendency to form an close space.