The problem for the laminar free convection from a horizontal isothermal infinite strip facing downward is studied by the momentum‐integral method within the boundary‐layer approximations when a vertical magnetic field is imposed on the plate. The solutions are presented in an integral form for the case when the induced magnetic field is negligible. The detailed calculations for Pr = 0.025 and 1 are compared with those of the nonmagnetic case. It is found that the boundary‐layer thickness increases and the Nusselt number decreases with the increase of the magnetic field.