The behavior of carbon particles was numerically studied for various sizes (0.01-5 mum) in the air on which a gradient magnetic field was imposed. One thousand particles were released randomly in one of the vertical cylindrical cross sections whose height is equal to its radius. An electric coil was placed coaxially at the bottom level of the enclosure to produce a magnetic field. The Brownian motion was taken into consideration and the Langevin equation was solved for the particle motion. The effect of the magnetic field on the particle movement increased with the particle size, but the Brownian motion decreased. For those particles with diameter d less than or equal to 1 mum, the Brownian motion was so strong that the magnetic and gravitational fields had almost no effects on the particle behavior. For the particles of d greater than or equal to 1 mum, the Brownian motion could be ignored mostly, and the particles accumulated at the bottom or upper walls of the enclosure depending on the magnetic strength.