The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45 degrees discrete V-finned tapes (DFT). The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000. The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated. Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively. Several V-finned tape characteristics were introduced such as fin-to duct-height ratio or blockage ratio (R-B = e/H = 0.075, 0.1, 0.15 and 0.2), fin pitch to duct height ratio (R-P = P/H = 0.5, 1.0, 1.5 and 2.0) and fin attack angle, alpha = 45 degrees. The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of RB but the decrease of RP. The inserted square-duct at RB = 0.2 and R-P = 0.5 provides the highest heat transfer and friction factor while the one with R-B = 0.1 and R-P = 1.5 yields the highest thermal performance. Also, a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.