In the present study, the natural convection of alumina-water nanofluid in an inclined C-shaped enclosure under the magnetic field was investigated. The inside walls are at a constant cold temperature. The right walls are adiabatic, where the other walls of cavity are heated with a constant heat flux. The results are based on the cooling optimization of heat walls using nanofluid under the magnetic field for inclination angles from 0 degrees to 90 degrees. The governing equations are solved using finite volume formulation and the SIMPLE algorithm is used for pressure-velocity coupling. The obtained results indicate that the influence of nanofluid on the average Nusselt number (Nu(av)) decreases by increasing Hartmann number (Ha). The Nuav decreases by increasing nanoparticles volume fraction up to Ha of 60 at Ra = 10(5). The effect of solid volume fraction can be neglected at Ha greater than 60. The results demonstrated that the increase in cavity aspect ratio (AR), nanoparticles volume fraction and inclination angle (a) values resulted in increasing of Nu(av). The influence of alpha on the heat transfer decreases by enhancement in AR. The nanofluid does not significantly affect the heat transfer by increasing a values. The minimum Nusselt number took place at AR = 0.2 and alpha = 45 degrees. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.