Nanofluids are considered to offer important advantages over conventional heat transfer fluids. The effects of alumina nanoparticles on convective heat transfer through a circular tube with a constant heat flux condition in a laminar flow regime have been numerically studied. The effects of nanoparticles concentration and Reynolds number are investigated in three types of base heat transfer fluids, which are pure water, water-ethylene glycol mixture and pure ethylene glycol. In general heat transfer enhancement, increase considerably with an augmentation of the Reynolds number. Also heat transfer increase with increasing the particle concentration.