In this paper, determination of heat transfer rate and Nusselt number in the thermal entry region in ducts is considered. Alternative relations are derived to calculate the rate of heat transfer and Nusselt number. An approximate equation is proposed for the calculation of mean and local Nusselt number in all range of the axial distance in the thermal entry region in ducts with constant surface temperature. And the use of this correlation is extended to predict mean and local Nusselt number in the thermal entry region in tubes with constant wall heat flux. Comparisons are made with available exact solutions. It is shown that the proposed equation yields quite accurate results in all range of the axial distance in thermal entry region. © 2004 Elsevier Science Ltd.