In this study, forced convection of nanofluid flow in various channel geometries with a hydraulic diameter of 16 mm and length of 1.5 m under laminar flow condition has been investigated numerically. Constant heat flux of 6 kW/m2 has been applied on to the surfaces of the channels. Fe3O4/water nanofluid has been used in the analyses to enhance the convective heat transfer of the base fluid. Analyses have been performed for Reynolds numbers between 500≤Re≤2000, and for volume concentrations of nanoparticles between 1% and 5% in cylindrical, square, rectangle, and triangle cross-sectioned channel geometries. The finite volume discretization method has been used to solve the governing equations. The effects of some parameters; Reynolds number, nanoparticle volume fractions, channel geometries on the average Nusselt number, Darcy friction factor and entropy generation have been investigated in detail. The results indicate that nanofluid offers further convective heat transfer enhancement according to base fluid and cylindrical cross-sectioned channel gives the best heat transfer performance among other cross-sectioned channel geometries. Using water as a working fluid, cylindrical cross-sectioned channel geometry gives the highest heat transfer rate among other channel geometries, whereas triangle one gives the lowest. Cylindrical cross-sectioned channel geometry offers up to 77.6% enhancement compared to triangle cross-sectioned channel geometry for the same hydraulic diameter and same heat flux. However, triangle cross-sectioned channel geometry has highest convective heat transfer increment ratio (4.12%) for changing working fluid as water to nanofluid. Also, some new Nu correlations based on the channel geometries and nanoparticle volume fractions were proposed in the present study.