Natural convection in a square enclosure filled with water-Al2O3 and water-TiO2 nanofluids is studied numerically. Upper, and lower surfaces are insulated, left wall is at high temperature, while right wall and two heat sinks vertically attached to the horizontal walls of the enclosure are kept at low temperature. A uniform magnetic field is applied in a horizontal direction. Steady state laminar regime is considered. The transport equations for continuity, momentum and energy are solved. The numerical results are reported for the effect of solid volume fraction, Rayleigh number, and Hartmann number on the Iso-contours of streamline and temperature. Also, the results for average Nusselt are presented for various parametric conditions. In addition, heat sinks length is investigated for different configurations. This study is presented in the following ranges, Rayleigh number from 10³ to 10⁵, Hartmann number from 0 to 60, and solid volume fraction from 0 to 0.06, while the Prandtl number representing water is kept constant at 6.2. The results show that heat transfer rate decreases with the increase of Hartmann number and increases with the rise of Rayleigh number and with the increase of the nanoparticles solid volume fraction. Also, results show that the heat sinks length have great effect on heat transfer rate depending on the Hartmann number, Rayleigh number and nanoparticle solid volume fraction. In addition, water-Al2O3 has better effect on convective heat transfer than water-TiO2 nanofluid.