In this paper, the effect of water-based Al2O3 nanofluids as working fluid on the thermal performance of a flat micro-heat pipe with a rectangular grooved wick is investigated. For the purpose, the axial variations of the wall temperature. the evaporation and condensation rates are considered by solving the one-dimensional conduction equation for the wall and the augmented Young-Laplace equation for the phase change process. In particular. the thermophysical properties of nanofluids as well as the surface characteristics formed by nanoparticles Such as a thin porous coating are considered. From the comparison of the thermal performance using both DI water and nanofluids. it is found that the thin porous coating layer formed by nanoparticles suspended in nanofluids is a key effect of the hear transfer enhancement for the heat pipe using nanofluids. Also, the effects of the Volume fraction and the size of nanoparticles oil the thermal performance are studied. The results shows the feasibility of enhancing the thermal performance up to 100% although water-based Al2O3 nanofluids with the concentration less than 10% is used as working fluid. Finally, it is shown that the thermal resistance of the nanofluid heat pipe tends to decrease with increasing the nanoparticle size. which corresponds to the previous experimental results. (C) 2009 Elsevier Ltd. All rights reserved.