© Shiraz University.Three-dimensional laminar flow and heat transfer characteristics in smooth hexagonal ducts with equal sides have been numerically investigated in the Reynolds number range from 300 to 2000. The numerical solutions are obtained for both axially and peripherally constant wall temperature (T) and heat flux (H2) thermal boundary conditions for five different values of the duct angle (θ = 30, 45, 60, 75, and 90°). Local Fanning friction factor and Nusselt number are obtained along the duct. Hydrodynamic and thermal entrance lengths have been determined. The accuracy of the results obtained in this study is verified by comparing the results with those available in the literature. Results show that duct geometry plays an important role on both flow and heat transfer characteristics. It is seen that the thermal entrance length for H2 boundary condition is greater than that for T boundary condition. Minimum hydrodynamic and thermal entrance lengths are obtained for θ = 60°, regular duct. New correlations, important for the design of thermal equipment, are presented for the hydrodynamic and thermal entrance lengths, friction factor, and Nusselt number for 30° ≤ θ ≤ 90° and 300 ≤ Re ≤ 2000.