The friction and heat transfer characteristics of single phase liquid flow through two multi-port extruded (MPE) micro-tubes were investigated by experiments and theoretical analysis. The cross-section geometries of two tubes were rectangular (approximate square) and circular, and the hydraulic diameters were 0.715 and 0.86 mm respectively. The working fluid was ethanol and its Reynolds number was in the range of 50-2400. The inlet temperature changed from 5 to 45 degrees C and the heat flux ranged from 3 to 9 kW/m(2). The measurements of pressure drop were performed under isothermal and non-isothermal (constant heat flux) conditions. The effects of different boundary conditions on the simultaneous pressure drop and heat transfer characteristics were studied. The results showed that the Nusselt numbers decrease with the increase of inlet temperature and heat flux. Furthermore, the scaling effects of conjugate heat transfer, entrance effects and temperature dependent viscosity variation on the convective heat transfer are significant. Based on the experimental data, new correlations for the Nusselt numbers were obtained considering the scaling effects for the multi-port tubes. (C) 2013 Elsevier Ltd. All rights reserved.