Nanofluids are the combination of a base fluid with nanoparticles with sizes of 1-100 nm. In order to increase the heat transfer performance, nanoparticles with higher thermal conductivity compared to that of base fluid are introduced into the base fluid. Main parameters affecting single-phase and two-phase heat transfer of nanofluids are shape, material type and average diameter of nanoparticles, mass fraction and stability of nanoparticles, surface roughness, and fluid inlet temperature. In this study, the effect of inlet temperature of deionized water/alumina (Al2O3) nanoparticle nanofluids was both experimentally and numerically investigated. Nanofluids with a mass fraction of 0.1% were tested inside a microtube having inner and outer diameters of 889 and 1,067 mu m, respectively, for hydrodynamically developed and thermally developing laminar flows at Reynolds numbers of 650, 1,000, and 1,300. According to the obtained numerical and experimental results, the inlet temperature effect was more pronounced for the thermally developing region. The performance enhancement with nanoparticles was obtained at rather higher Reynolds numbers and near the inlet of the microtube. There was a good agreement between the experimental and numerical results so that the numerical approach could be further implemented in future studies on nanofluid flows.