The experimental and numerical analysis of combined forced and free convection heat transfer of a non-Newtonian fluid in a horizontal annular duct is presented. The flow is laminar and Prandtl and Boussinesq hypotheses are adopted. The outer and inner cylinders are heated uniformly with a constant heat flux density. At the inlet of the annular duct, the flow is fully developed and the temperature profile is uniform. The governing equations are solved numerically using finite differences. The variation of the rheological properties with temperature is taken into account. Near the entrance, forced convection is the dominant mechanism. The core flow is decelerated because of the decrease of the consistency K as the temperature T increases near the heated walls. Simultaneously, the downstream flow between the two cylinders caused by the displacement of the secondary boundary layer induces an acceleration of the flow in the lower part of the annular duct and a deceleration in the upper part. Further downstream, the fluid warms up and the buoyancy force effect becomes strong enough to overcome the forced flow. The critical Cameron number X + c , above which the convection mechanism becomes dominated by natural convection is determined using scaling analysis. The results are found to agree well with the numerical solution. For X + < X + c and for a large Péclet number, an asymptotic solution is obtained by perturbing the forced convection solution. However, far from the entrance region, the thermal stratification induced by buoyancy force combined with the variation of K with T lead to another flow reorganization. There is an acceleration of the flow in the upper half part of the annular duct and a deceleration in the lower half one.