In this paper, heat dissipation effect of a ferrofluid on natural convection flow in a cavity with linearly temperature distribution at the presence of an external magnetic source has been analyzed with Lattice Boltzmann method (LBM). The cavity is filled with kerosene as the carrier fluid and nanoscale ferromagnetic particle of cobalt. This study has been performed for the pertinent parameters in the following ranges: the Rayleigh number of base fluid, Ra = 103-105, the volumetric fraction of nanoscale ferromagnetic particle between 0 and 3% and the size of the nanoscale ferromagnetic particle is fixed at 45 nm. Results show that the heat transfer decreases by the increment of the nanoscale ferromagnetic particle volume fraction for various Rayleigh numbers. The external magnetic source influences the nanoscale ferromagnetic particle at Ra = 105 more than other Raleigh numbers as the least effect is observed at Ra = 103.