The Galerkin integral method is applied to calculate the friction factor for laminar flow in rhombic ducts imposed by constant wall temperature at the boundaries. The local Nusselt number, Nu x,T , and thermally fully developed Nusselt number, Nu T , are also computed. The flow is assumed to be hydrodynamically fully developed and laminar. The fluid is Newtonian an has a large Peclet number. The thermally fully developed Nusselt number, Nu T , can be obtained when the local Nusselt number asymptotically approaches a constant value. It is found that a larger amount of heat transfer occurs at the entrance region of the rhombic duct. A ten-term solution provides accurate predictions in the values of C f Re for a rhombic duct having a larger angle θ . However, for a duct with a smaller angle, a larger number of basis functions must be used. It is also found that the shape of the isothermal profile is an ellipse when the local Nusselt number approaches a constant value.