A finite volume based three dimensional numerical analysis is performed on the effect of angle of inclination of the external magnetic force on entropy generation due to natural convection inside the cubical cavity filled with CNT - water nanofluid. The governing equations are numerically solved by vorticity - vector potential formalism.  The vertical walls of enclosure are differentially heated and the horizontal walls are adiabatic. The effect due to Rayleigh number (10³ ≤ Ra ≤ 10⁵), Hartmann number (0 ≤ Ha ≤ 100), angle of inclination of external magnetic field (0° ≤ α ≤ 90°) and volumetric fraction (0 ≤ φ ≤ 0.05) of CNT particles are investigated.  The results of irreversibility process with single phase model are illustrated with entropy generation and Bejan number.   The effect of angle of inclination is more pronounced when Rayleigh number is 10⁵.