A numerical simulation is developed to examine the secondary flow characteristics in a curved rectangular duct with external heating on the concave wall of the duct. The general governing equations of momentum, mass continuity and energy of the flow are non-dimensionalised and transformed into vorticity-stream function formulation. The equations are discretised using finite volume method and the dynamical variables are defined on a stagger grid. The resulting penta-diagonal system of linear algebraic equations is solved using Strongly-Implicit-Procedure. Numerical simulation is performed on ducts of aspect ratios between 1 to 8 and selected curvature. The external wall heating is varied up to 200 W/m2 and the numerical solutions are obtained for Dean number ranging from 20-500. Results are presented to illustrate the effects of Dean number, aspect ratio, buoyancy force and external heating on the flow parameters.