A two-dimensional thermal laminar natural convection of molten gallium in square enclosure isothermally heated and cooled from a side vertical wall was numerically computed in both time-periodic magnetic and gravity fields. The magnetic field is described by a sinusoidal time-varying equation. The thermal coefficient heat transfer has been calculated for various magnetic field strengths, but at a certain value of frequency and for different frequencies, but at a certain value of magnetic field strength. The results show that the effect of the time-periodic magnetic field on the flow field and heat transfer is lower than that of steady (DC) magnetic field effect at the same magnetic field strength.