By means of magnetization treatments at ambient temperature and elevated temperatures, the nano- and micron-bismuth ferrate/low density polyethylene (BiFeO/LDPE) dielectric composites are developed to explore the material processing method to modify the crystalline morphology, magnetic and dielectric properties. The magnetic field treatment can induce the dipole in the LDPE macromolecular chain which leads to preferred orientation of polyethylene crystal grains to the direction of the magnetization field. The surface morphology of the materials measured by atomic force microscope (AFM) implies that the LDPE macromolecular chains in BiFeO/LDPE composites have been orderly arranged and form thicker lamellae accumulated with a larger spacing after high temperature magnetization, resulting in the increased dimension and orientation of spherulites. The residual magnetization intensities of BiFeO/LDPE composites have been significantly improved by magnetization treatments at ambient temperature. After this magnetization at ambient temperature, the of nano- and micron-BiFeO/LDPE composites approach to 4.415 × 10 and 0.690 × 10 emu/g, respectively. The magnetic moments of BiFeO fillers are arranged parallel to the magnetic field direction, leading to appreciable enhancement of the magnetic interactions between BiFeO fillers, which will inhibit the polarization of the electric dipole moments at the interface between BiFeO fillers and the LDPE matrix. Therefore, magnetization treatment results in the lower dielectric constant and higher dielectric loss of BiFeO/LDPE composites. It is proven that the magnetic and dielectric properties of polymer dielectric composites can be effectively modified by the magnetization treatment in the melt blending process of preparing composites, which is expected to provide a technical strategy for developing magnetic polymer dielectrics.