In this paper, the viscosity of Al, In, Sn, and Bi melts under different magnetic field intensities has been investigated by using a torsional oscillation viscometer. The experimental results show that the viscosity curves fit well with the Arrhenius formula under the magnetic field and the viscosity of all melts increases as the horizontal magnetic field intensity enhances. At the same conditions, the viscosity change rates of Al and In are obviously larger than that of Sn and Bi which possess covalent bonds. And the viscosity change rate of the lightest element Al is the largest in the samples.