The effect of an externally imposed magnetic field on the fluid flow in a rectangular cavity are studied numerically. The power-law weighted upwind method is used for the numerical solution of the vorticity transport equation and the energy equation. And the successive over-relaxation method is applied to solve the Poisson equation. It is found that the magnetic field perpendicular to the gravitation is more effective to delay and suppress the natural convection than the magnetic field parallel to the gravitation. The higher the imposed magnetic field is, the greater is the reduction in the absolute value of the velocity, and the less the convection plays a role in the temperature distribution. The direction of the magnetic field does not give as much effect in suppressing the natural convection as the magnetic field strength when it is stronger than 0.2 tesla.