ABSTRACT:- In airlift reactors transport phenomena are achieved by pneumatic agitation and circulation occurs in a defined cyclic pattern through a loop. In the present work, the effect of geometrical relations on gas holdup and liquid velocity, and consequently on the gas-liquid mass transfer coefficient, was studied in a 15-liter airlift bioreactor with Ad/Ar = 0.224, Ad : downcomer cross-sectional area, and Ar : riser cross-sectional area. Measurements of the volumetric oxygen transfer coefficient (KLa) were taken in a raw water-air system using a sulfite oxidation method for depleting the oxygen to zero. The unsteady state concentration of oxygen is then measured using DO probe. Different conditions were examined by varying parameters such as superficial air velocity in the riser (Usg), using two types of air sparger which have a remarkable effect on KLa values. The effect is due to their influence on gas holdup and liquid velocity, consequently affecting KLa. Superficial air velocity in the riser (Usg) ranged from 0.03 to 0.06 m.s-1 and KLa varied between 0.01 to 0.07 s-1. Empirical equations were obtained between KLa and Usg as: KLa = 0.28 Usg 0.53 for cross sparger KLa = 0.58 Usg 0.86 for o-ring spargerKeywords:- Airlift, Bioreactor, hydrodynamics, oxygen mass transfer rate.