The positron annihilation lifetime spectroscopy(PALS), coincidence doppler broadening spectroscopy(CDBS) and x-ray diffraction(XRD) are measured in the Fe3O4 . The aim is to ascertain how pressure and annealing temperature influence the properties of Fe3O4 nanoparticles, such as the change of phase, electronic structure, defect and distribution of electronic momenta. The results indicate that the defect concentration of Fe3O4 nanoparticles increases with the increase of pressure, but the phase and defect type do not change. The phase type, and the guantity and type of defect change significantly as the annealing temperature varies. The phase transition of the Fe3O4 wafer pressed in atmosphere occurs when the annealing temperature is between 350 ℃ and 500 ℃. The grain size of Fe3O4 increases with the increase of the annealing temperature, which leads to the decrease in boundary defect and parameter S but it is contrary to the ratio curve.