© 2016, Shiraz University.Three-dimensional turbulent-forced convection flow, heat transfer and second-law analysis in a circular duct having single baffle have been examined numerically under uniform constant wall heat flux boundary condition at steady state. Baffle is attached in the entrance, middle and exit regions of the test section. ANSYS Fluent 15 which uses finite-volume method has been employed for numerical analysis. The effects of Reynolds number Re changing from 3000 to 50,000 and dimensionless position of baffle S/D = 1, 16.1 and 25 are investigated for Prandtl number of Pr = 0.7 and baffle angle of α = 90°. It is seen that circular duct with single baffle has a higher Nusselt number, friction factor and entropy generation rate compared to the circular duct without baffle. It is also seen that the duct with baffle in the inlet region has a higher value of Nusselt number and friction factor. The duct having baffle in the middle region has a maximum thermal performance and low entropy generation rate. The accuracy of the results is validated by comparing the obtained results with the results of smooth duct.