Abstract: Fe3O4 magnetic fluids of two different shapes of sphere and square were prepared by two different methods. The samples were characterized with XRD, TEM, and SEM. Fe3O4 magnetic fluids of two different morphologies were used for the magnetic particle detection of the artificial test block. The results show that during the magnetization process, sphere Fe3O4 of water based magnetic fluids was gathering to the defect leakage magnetic field faster than that of square one and crack indication was exactly clear. After withdrawing the external magnetic field, the defect indications were destroyed by the disturbance of the fluids. The defect indications by using pure spherical nano Fe3O4 magnetic fluids were not as clear as that with using square nano Fe3O4 magnetic fluids.