A numerical study was made of natural convection of air (paramagnetic fluid) in a cubic enclosure under constant gravity and time-periodic magnetic field. In the cube, the two opposite vertical sidewalls are maintained at different temperatures and the other walls are thermally insulated. In this basic layout, the magnetic field is applied externally by placing two sets of electrical coil; left and right sides of the cube. The currents in the coils are activated in an alternating fashion. The computed results revealed the existence of resonance, and the resonance frequency is characterized by the basic mode of internal gravity oscillations. The amplification characteristics of the amplitude of heat transfer in the interior are linearly dependent on the magnitude of magnetizing force.