Steady and unsteady state oxygen concentration distributions in the liquid and gas phases along the axial direction of different airlift bioreactor scales have been simulated for various gas flow rates and oxygen consumption rates by applying the axial dispersion model to the riser and the downcomer, and a complete mixing model for the top (separator) and the bottom sections of the bioreactor. The results show that the dissolved oxygen concentration is very low at the lower part of the downcomer when the rate of oxygen consumption by microorganisms is very high. Furthermore, the shorter (small) bioreactor shows relatively more uniform axial dissolved oxygen concentrations than the longer (large) bioreactor, due to the effect of the hydrostatic pressure along the bioreactor. One of the most important geometric factors for mass transfer is the reactor height, which dominates the mean pressure and thus influences the saturation concentration and mass transfer driving force. The presented model can be applied for modeling and scale-up of practical airlift bioreactors.