Experimental investigation was performed on the mixed convection heat transfer of thermal entrance region in an inclined rectangular duct for laminar and transition flow. Air flowed upwardly and downwardly with inclination angles from -90 degrees to 90 degrees. The duct was made of duralumin plate and heated with uniform heat flux axially. The experiment was designed for determining the effects of inclination angles on the heat transfer coefficients and friction factors at seven orientations (theta = - 90 degrees, -60 degrees, -30 degrees, 0 degrees, 30 degrees, 60 degrees and 90 degrees), six Reynolds numbers (Re approximate to 420, 840, 1290, 1720, 2190 and 2630) within the range of Grashof numbers from 6.8 x 10(3) to 4.1 x 10(4). The optimum inclination angles that yielded the maximum heat transfer coefficients decreased from 30 degrees to -30 degrees with the increase of Reynolds numbers from 420 to 1720. The heat transfer coefficients first increased with inclination angles up to a maximum value and then decreased. With further increase in Reynolds numbers, the heat transfer coefficients were nearly independent of inclination angles. The friction factors decreased with the increase of inclination angles from -90 degrees to 90 degrees when Reynolds numbers ranged from 420 to 1290, and independent of inclination angles with higher Reynolds numbers. (C) 2008 Elsevier Ltd. All rights reserved.