Natural ventilation of air from a vertical open cylinder with a magnetic field was numerically computed. The upper half of a cylinder wall was uniformly heated and the lower half of a cylinder thermally insulated. One turn electric coil was placed at either Z = 2.5, 5 or 10 from the bottom opening to study its effect on the rate of ventilation. Sample computations were carried out for Pr = 0.7, Ra* = 1000, and γ = 100, 1000 where γ represents the strength of magnetic field. For the coil at Z = 2.5, the effect was slight but at Z = 5, the magnetic field enhanced the convection of air near the heated wall extensively to give the annular velocity profile. For the coil at Z = 10, at the exit, the flow was reversed to downward at γ = 1000. Magnetic field can control the convection of air extensively.