In this paper, a parallel plate structure for the magnetic capacitor applications is presented, which consists of hybrid materials of Fe3O4 nanoparticles with polydimethylsiloxane (PDMS) as the dielectric medium. By changing the nanoparticle sizes and concentrations in PDMS, the magnetic-capacitance effect of the designed structure is investigated, and some key factors which may affect the performances are studied. It can be concluded from the results that a clear magnetic-capacitance coupling effect is observed by putting the designed Fe3O4 nanoparticles and PDMS hybrid material in or out of a magnetic field. Meanwhile, as we increase the concentration of the nanoparticles, an increase of capacitance variation may be observed. If the nanoparticle sizes are bigger than the critical dimension of the super-paramagnetic effect, the capacitance variations is increased as the nanoparticle size increases.