Natural convection heat transfer in a square enclosure with a horizontal baffle at the center line of left wall containing Al2O3-water in the presence of a magnetic field is investigated numerically. The top and bottom horizontal walls are adiabatic. The left wall and the baffle are maintained in constant temperature Thand the right wall is maintained in constant temperature Tc (Tc <Th). Discretized equations are solved using the SIMPLE algorithm. The numerical simulations have been carried out to determine the effect of parameters in the following ranges: Rayleigh number, Ra=103 to 106, nanoparticles volume fraction between =0 to 5%, Hartmann number varied from Ha=0 to 60 and baffle length (L1=0 to 0.5L). The results show that the heat transfer rate increases with increasing Rayleigh number, but with increasing Hartmann number decreases. Also, increasing the baffle length enhances the heat transfer rate into the enclosure.