Experiments of flow boiling heat transfer were conducted in four horizontal flattened smooth copper tubes of two different heights of 2 and 3 mm. The equivalent diameters of the flattened tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids were R22 and R410A. The test conditions were: mass velocities from 150 to 500 kg/m(2) s, heat fluxes from 6 to 40 kW/m(2) and saturation temperature of 5 degrees C. The experimental heat transfer results are presented and the effects of mass flux, heat flux, and tube diameter on heat transfer are analyzed. Furthermore, the flow pattern based flow boiling heat transfer model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R Thome, Investigation of flow boiling in horizontal tubes: Part I - A new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955-2969; L. Wojtan, T. Ursenbacker, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II - Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970-2985], using the equivalent diameters, were compared to the experimental data. The model predicts 71% of the entire database of R22 and R410A +/- 30% overall. The model predicts well the flattened tube heat transfer coefficients for R22 while it does not predicts well those for R410A. Based on several physical considerations, a modified flow boiling heat transfer model was proposed for the flattened tubes on the basis of the Wojtan et al. model and it predicts the flattened tube heat transfer database of R22 and R410A by 85.8% within +/- 30%. The modified model is applied to the reduced pressures up to 0.19. (C) 2009 Elsevier Ltd. All rights reserved.