A numerical study of flow field and heat transfer in a heated circular tube with an inner tube inserted has been investigated. Due to the insertion of the inner tube, part of the fluid is deviated from the original path to the heated tube which results in the increase of heat transfer rate from the heated wall. The Projection method is adopted to predict the flow field and three different types of inner tube, (1) straight tube, (2) contraction-enlargement tube and (3) enlargement-contraction tube, are considered. The calculations are performed for Reynolds numbers of 100, 500, 1000, and Prandtl numbers of 0.1, 0.7, 7 and 10. The results show that, except at very low Peclet number, the heat transfer rate of the heated tube increases when an inner tube is inserted. About 50% increase in heat transfer rate can be achieved under the condition of constant pumping power evaluation and twice the heat transfer rate than for the empty tube can be achieved under the condition of constant flow rate evaluation.