An experimental investigation on the convective heat transfer characteristics in the developing region of tube flow with constant heat flux is carried out with alumina-water nanofluids. The primary objective is to evaluate the effect of particle size on convective heat transfer in laminar developing region. Two particle sizes were used, one with average particle size off 45 nm and the other with 150 nm. It was observed that both nanofluids showed higher heat transfer characteristics than the base fluid and the nanofluid with 45 nm particles showed higher heat transfer coefficient than that with 150 nm particles. It was also observed that in the developing region, the heat transfer coefficients show higher enhancement than in the developed region. Based on the experimental results a correlation for heat transfer in the developing region has been proposed for the present range of nanofluids. © 2009 Elsevier Ltd. All rights reserved.