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A numerical simulation is presented to describe the secondary flow characteristics in the flow through curved rectangular ducts that are heated on the outer (concave) wall. The buoyancy body forces due to external heating are incorporated in the formulation. The governing equations of momentum, mass continuity and energy of the flow are non-dimensionalised and transformed into vorticity-streamfunction. The equations are discretised using a finite volume method and the dynamical variables are defined on a staggered grid. The resulting penta-diagonal system of linear algebraic equations is solved using the Strongly-Implicit-Procedure. Numerical computations are performed for the flow through rectangular ducts of aspect ratios 1 to 8 and for the Dean number lying in the range 20 to 500. The external wall heat fluxes of 0 to 200 W·m−2 are considered. The computation accurately captures the influence of external heating on the secondary vortex motion and, predicts the occurrence of Dean vortices in non-isothermal flow through the curved ducts. With wall heating, the flow pattern rapidly deviate from the symmetrical vortex motion observed under isothermal conditions. The formation of Dean vortices still takes place in the flow although it is postponed to higher flow rates where the flow symmetry is gradually re-established. The number of Dean vortices formed in the flow is strongly influenced by the aspect ratio of ducts. Convective heat transfer is significantly enhanced by the secondary flow particularly when the Dean vortices emerge at the outer wall. The predicted characteristics of secondary flow and heat transfer conform to and agree well with the available experimental data. [Copyright &y& Elsevier]