The purpose of this study is to modify the Fluidized Bed Process. Placing various particle diameters of granular activated carbon, (#20∼#30, #30∼#40, #40∼#50) as a medium in an aeration tank, in which microorganisms can attach and grow, improves the biomass concentration, volumetric loadings and removal efficiency. The continuous supply of substrates allows the thickness of bio-film on the medium to be shifted to a proper state. This is due to a friction force effect between the media and the shear force of the agitated flow. After studying the comparisons, it is concluded that the middle particle size (#30∼#40) is the best of the three particle diameters as a medium, in that it achieves a high biomass concentration, stable treatment and higher efficiency.