Fluidization for plasma treatment of particles having high density such as ferrite is studied under the condition of reduced pressure. These particles are observed to be fluidized at about 100 Pa. The relation between minimum fluidization velocity and the freeboard pressure in the fluidized bed is investigated. The minimum fluidization velocity can be correlated by the equation in which the Knudsen term is added to the viscosity term under atmospheric conditions. The charge of ferrite can be changed by the plasma treatment of gas (such as oxygen or hydrogen) or monomer (e.g. acrylic acid). The charge can be qualitatively predicted by considering the series of frictional electrification.