The purpose of this study is to numerically investigate the laminar fluid flow and heat transfer in trapezoidal ducts filled with an Al2O3-water nanofluid under constant heat flux boundary conditions. A numerical study is carried out using the commercial code ANSYS Fluent 17.0. The examined parameters are the Reynolds number (Re), side angle of a trapezoidal duct (alpha), and the volume fraction of particles in the nanofluid (vof). Study is carried out at Re = 300-750, alpha = 30-85 degrees, and vof = 0-4%. The heat transfer coefficient, Nusselt number, pressure drop, friction factor, and the performance index are examined. Fully developed results are compared with the literature data, and it is seen that they are in good agreement with the present results. The results obtained show that the heat transfer coefficient and pressure drop increase with the volume fraction of the nanoparticles in the nanofluid. It is also seen that the heat transfer coefficient and pressure drop increase with the Reynolds number and side angle at a given volume concentration. The performance index is greater than unity in all numerical studies. The performance index increases with the side angle of the duct and volume fraction of nanoparticles in the nanofluid but decreases with an increasing Reynolds number. The performance index shows that the use of nanoparticles in water for laminar flow in trapezoidal ducts is advantageous.