The sub-micron (of the order of 150 nm) thick core-shell composite fibers of magnesium ferrite-polyvinylidene fluoride are prepared by electrospinning. The loading of magnesium ferrite is varied from 1 to 10 wt%. The study results by X-ray diffraction, scanning electron microscope, and infra-red spectroscopy indicate the formation of core-shell structure and an enhancement in the amount of β-phase compared to α-phase in the polyvinylidene fluoride. The particle size of the magnesium ferrite in the fiber is evaluated to be 30 nm. The low frequency dielectric studies indicate that the addition of the magnesium ferrite increases the polarization resulting in the increase in the dielectric constant but decreases the dielectric loss. The magnetization measurements indicate an increased value of coercivity compared to bulk due to the nano-size of the magnesium ferrite. The microwave absorption at the ferromagnetic resonance increases with the increase in the concentration of magnesium ferrite. The resonance field is found to vary with the loading of MFO. © 2014 The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg.