Absract Energy costs have soared rapidly in the last decade and there is tremendous need for new kinds of heating/cooling fluids, to improve heating system performance and reduce equipment size and energy consumption. Study of thermal characteristics for laminar fully developed flow thruogh conduits with different cross-sections is significant in design of heat exchangers. The primary objective of the heat exchanger designer is to work with geometeries that yield (1) a high value of heat transfer area to volume ratio (2) a high value of heat transfer coefficient and (3) a corresponding low value of friction factor. Nanofluids are a new phenomenon of working fluids with high thermal potential used for industrial applications such as construction of compact heat exchangers. It seems we can achive the primary objective of the heat exchanger designs by using of non-circular cross sectio instead of circular cross section and using of nanofluids instead of conventional fluids. Therfore in this study, we investigate experimental laminar forced convective heat transfer of nanofluid through conduits with different cross sections. We study the effects of different parameters such as cross section shape(circle, square, rectangle and triangle), Reynolds number(500-2400) and volume concentration of nanoparticles in suspension(0.2% and 0.5%) on heat transfer by designing and assembling of an experimental aparatus. Very good agreement between experimental data and Seider-Tate equation results for distilled water were obtained which emphasizes the accuracy of experiments. Experimental results indicate that the Nusselt number increases with increment of Reynolds number and volume fraction of TiO 2 nanoparticles. For a specific range of Reynolds number, the Nusselt numbers of Nano fluid are higher than those of the base fluid for all cases. This illustrates that adding a few amount of nano powder can improve Nusselt number in conduits. The average enhancement of 5.84%, 7.37%, 7.21 and 7.18 for volume fraction of 0.5% and average enhancement of 2.74%, 3.17% , 3.20 and 3.36% for volume fraction of 0.2% were observed in conduits with circular, square, rectangle and triangle cross sections, respectively. Experimental data express that Nusselt number of circular case for all range of Reynolds number at specific volume fraction is higher than that of other three cases and Nusselt number of square and rectangle are higher than that of triangle case. For example at Reynolds number of 2000 for nanofluids at 0.5% volume fraction of TiO 2 nanoparticles, the Nusselt number is 10.41, 10.08, 9.91 and 8.85 for circular, square, rectangle and triangle cross section, respectively. For a specific Reynolds number and volume fraction of TiO 2 nanoparticles, the Nusselt number of fluids flow through conduit with circular cross sections is more than that of conduits with square, rectangle and triangle cross sections. The relative enhancement of Nusselt number of working fluids flow through conduit with circular cross section is less than that of conduits with square, rectangle and triangle cross section. This behavior indicates that heat lost in sharp angles is reduced. Keywords: heat transfer, TiO 2 /water nanofluid, different cross section, uniform wall temperature