Natural convection of liquid metal in a cubic enclosure heated from one vertical wall and cooled from an opposing vertical wall is studied under a horizontal lateral magnetic field parallel to the heated and cooled walls. The Seebeck effect is further considered for the present system. The Seebeck effect means the electric current induced having temperature difference in the two metals between two points. This effect may be induced for the liquid and solid boundaries of metals. This system was modeled and numerically computed. When the Seebeck effect is not considered, the Lorentz force increases with the Hartmann number, Ha, to decrease the average Nusselt number. When the Seebeck effect was considered, the average Nusselt number at Ha = 200 decreased. However at Ha = 400, the average Nusselt number increased with the Seebeck effect. This peculiar characteristic is due to the complicated convection of the Seebeck effect on the heat transfer surface.