The effects of natural convection flow in presence of magnetic force in a two-dimensional open square cavity. Firstly, a two-dimensional steady state natural convection flow in an open square cavity is considered. A heated circular cylinder is placed at the centre of the cavity where the left sidewall to the cavity is heated by a constant heat flux. The top wall is kept cool and the bottom wall is kept at a hot temperature. The fluid is concerned with different Prandtl numbers.The significant findings of this work are that for all cases of Ha and Ra the isothermal lines concentrated to the right lower corner of the cavity and recirculations are created around the cylinder counterclockwise and one small vortex has been created above the cylinder in the cavity. The recirculation region are increasing with increasing Ra. The result of this work is a good agreement with the existing heat transfer theory. In the line graphs it is observed that heat flux increases for higher Prandtl numbers. Here the total heat flux decreases mainly for higher inclination angles. This is because of the opening side become the upper side and the temperature difference become less than those of previous.