In the present paper, a systematic investigation on numerical modeling of variable-property microchannel flows with electro-thermo-hydrodynamic interactions is presented. General variable-property electro-thermal flow model consisting of mass, momentum, energy, electric potential and ions transport equations in dimensionless form is formulated and employed to study transport phenomena in microchannel flows. The major concerns are: (1) evaluation of the model approximations such as equilibrium ion distribution, constant-property and/or small zeta potential for non-overlap electric double layer (EDL); (2) the electro-thermo-hydrodynamic interactions under conditions of strong EDL effects, and (3) differences in microchannel flow characteristics at driving conditions of constant pressure gradient (CPG) and constant flow rate (CFR). Variations of fluid, transport and electric properties with local temperature and/or ion concentration are considered. The present results demonstrate that the variable-property model with Poisson-Boltzmann potential is an appropriate approximation for transport phenomena in microchannel flows. The results also disclose that, compared to those in CPG flows based on the same Reynolds number, effects of temperature non-uniformity and variable-property are relatively more pronounced in the CFR flows. At either CPG or CFR condition, heat transfer performance changes little with the variations of electric and flow parameters in this low Reynolds number (Re = 0.1) flow; while electric and flow fields can be significantly affected by variation of the governing parameters. (C) 2007 Elsevier Ltd. All rights reserved.