This paper experimentally investigates the heat transfer of an equilateral triangular duct by employing an Al2O3/water nanofluid in laminar flow and under constant heat flux conditions to improve the heat transfer performance of this type of duct. The Nusselt numbers were obtained for different nanoparticle concentrations of the nanofluid at various Peclet numbers. The results show that the experimental heat transfer coefficient of Al2O3/water nanofluid is higher than that of distillated water. Also, the experimental heat transfer coefficient of Al2O3/water nanofluid is higher than the theoretical one. The experimental results also indicate that the heat transfer enhancement increases with increases in the nanofluid volume concentration and Peclet number.