A detachment of biofilm was investigated in an inverse fluidized bed biofilm reactor(IFBBR). The biofilm thickness, δ and the bioparticle density, ρ_pd were decreased by the increase of Reynolds number, Re and the decrease of biomass concentration, b_c. The correlations were expressed as δ=61.6+16.33b_c-0.04Re and p_pd=0.3+0.027b_c- 2.93x10 exp (-5)Re by multiple linear regression analysis method. Specific substrate removal rate, q was derived by F/M ratio and biofilm thickness as q=0,44+0.82F/M-5.1x10 exp (-4)δ. Specific biofilm detachment rate, b_ds, was influenced by F/M ratio and Reynolds number as b_ds=-0.26+0.26F/M+2.17x10 exp (-4)Re. Specific biofilm deachment rate in an IFBBR was higher than that in a FBBR(fluidized bed biofilm reactor) because of the friction between air bubble and the bioparticles.