The hydrodynamic characteristic performance of an internal draft tube inverse fluidized bed biofilm reactor was studied for the aerobic biodegradation of phenol (1,200 mg/l) using Pseudomonas fluorescens for various ratios of settled bed volume to reactor working volume (Vb/Vr) under batchwise condition with respect to liquid phase. The operating parameters, such as superficial gas velocity, phase hold ups, aspect ratio and bed height, were analyzed for different ratios of (Vb/Vr). The effect of biodegradation on synthetic phenolic effluent was determined from the reduction in chemical oxygen demand and phenol removal efficiency. The optimum value of (Vb/Vr)m was 0.20 for the optimal superficial gas velocity, Ugm=0.220 m/s with the COD reduction efficiency of 98.5% in 48 hours. The biomass and biofilm characteristics of P. fluorescens were determined for optimal hydrodynamic operating parameters by evaluating its biofilm dry density and thickness, bioparticle density, suspended and attached biomass concentration.