A numerical investigation is conducted on the turbulent flow and heat transfer characteristics for a set of equilateral polygonal ducts and rectangular ducts with different length-width ratios. All the ducts have the same hydraulic diameter as their characteristic length dimension in the Reynolds number. The flow is modeled as three-dimensional and fully elliptic by using the finite volume method and the standard k-ε turbulence model was adopted. The turbulent heat transfer performances have been thoroughly investigated. The results showed that for the equilateral polygonal duct, there is an appreciable reduction of the heat transfer performance in the corner region. The use of hydraulic diameter for noncircular ducts such as equilateral triangular duct will result in unacceptably large errors, of order of 30%, in turbulent heat transfer Nusselt number determined from the circular tube correlations. Based on the simulation results, a correction factor Cφ is proposed to ameliorate the previous correlations, and the errors in the prediction of turbulent heat transfer with the new correlation are within 7%. A new correlation is raised to predict the turbulent heat transfer Nusselt number for rectangular ducts with different length-width ratios. The deviations of the new correlation are within 10%.