Analyses were performed of the effect of Hartmann layers and side layers on heat transfer in laminar MHD flow in ducts and the dependence on the magnitude of the Hartmann number. Analytical and numerical results are presented for both fully developed and thermally developing cases. The presence of side layers in a rectangular duct usually increases the heat transfer coefficient on the side layer walls and decreases the heat transfer coefficient on the other two walls. For ducts with uniform thickness and conductivity on all walls, the studies show that a duct with higher conductance ratio gives higher average Nusselt number on the side wall. However, this behavior depends on the combination of Hartmann number and the conductance ratio. The heat generation inside the duct enhances the heat transfer coefficient.