This paper reports the numerical study of natural convection in a square enclosure filled with CNT-water nanofluid and exposed to a uniform external magnetic field. Heating is ensured by twothin fins. Using the control volume method, the effects of the fins position, their length and spacing as well as the solid volume fraction, the Rayleigh number and the Hartmann number on the thermal performance of the cavity were examined. The results obtained show that the heat transfer rate increases with the Rayleigh number, solid volume fraction and fins length; but decreases with Hartmann numbers. A comparison is also carried out between the results obtained from the Maxwell and Xue models. The results prove that the mean Nusselt number is higher based on the Xue model.