An on-line respiration meter was applied to measure the oxygen transfer coefficient (KLa) continuously in a completely mixed aeration tank. The uniqueness of this respiration meter lies in its ability to measure the actual respiration rate of activated sludge. The calculation of KLa can be carried out continuously with steady-state and discrete non-steady state equations using actual respiration rate and DO in the aeration tank. In an activated sludge pilot plant, the oxygen transfer coefficients were determined under constant and variable loading conditions using the above procedures. The relationship between KLa and air flow was found to be linear but dependent on the wastewater loading characteristic. Under variable loading conditions, the calculated DOs in the aeration tank using the obtained KLa's were more accurate than that using the KLa's estimated from the air flow rate function. The accuracy of separately assumed saturation DO in the activated sludge was confirmed from a linear relationship between DOs in the aeration tank and corresponding actual respiration rates at a constant KLa.