Abstract In this paper, effect of magnetic field on free convection heat transfer in an enclosure filled with nanofluid is studied. KKL (Koo-Kleinstreuer-Li) correlation is used for simulating effective thermal conductivity and viscosity of nanofluid. The inner cylinder is maintained at uniform heat flux and the outer cylinder has constant temperature. The governing equations are solved via Control Volume based Finite Element Method. The heat transfer between cold and hot regions of the enclosure cannot be well understood by using isotherm patterns so heatline visualization technique is used to find the direction and intensity of heat transfer in a domain. Effect of Hartmann number, volume fraction of nanoparticle, Rayleigh number and aspect ratio on streamline, isotherm and heatline are examined. The results show that as Hartmann number increases Nusselt number decreases while opposite trend is observed as nanoparticles volume fraction, Rayleigh number and aspect ratio increase. Domination of conduction mechanism causes heat transfer enhancement to increase. So enhancement in heat transfer increases with increase of Hartmann number and aspect ratio while it decreases with augment of Rayleigh number.