In this paper, turbulent convective heat transfer in a two-dimensional microtube with 10 mm diameter and variable length with constant heating temperature was numerically investigated. The governing (continuity, momentum and energy) equations were solved using the finite volume method with the aid of SIMPLE algorithm on FLUENT commercial code. Water-Al2O3 nanofluids with different volume fractions ranged from 1% to 4% were used. This investigation covers Reynolds number in the range of 10(4)-10(5). The results have shown that convective heat transfer coefficient for a nanofluid is enhanced than that of the base liquid. Wall heat transfer flux is increasing with the particle volume concentration and Reynolds number. Moreover, a study on microtube length influence on heat transfer was attempted and few correlations were established. (C) 2013 Elsevier Ltd, All rights reserved.