The behavior of carbon particles from 0.01 to 5 mu m diameters in natural convection flow field of air was numerically studied including the effect of the magnetic field and the Brownian motion. One thousand carbon particles were released randomly in a vertical cylindrical enclosure whose height is equal to its radius. The enclosure was heated from below and cooled from above to produce the bulk flow of the natural convection of air in the atmospheric condition. A coil with electric current was set coaxially at the enclosure bottom to produce a magnetic field. Sample computations are carried out for the enclosure of 3 cm high and maximum magnetic field is 2.8 T or less. The results show that the natural convection of air is enhanced by the magnetic field. With the increase in the magnetic strength, more particles go along the fluid streak lines. With the increase in the particle size, more particles get together at the vortex center and cluster. (c) 2005 Elsevier Ltd. All rights reserved.