A mathematical model for simulation of unsteady-state oxyden transfer in an external-loop airlift reactor was developed. The airlift reactor was represented by a number of interconnected tanks, each of which was assumed to be well mixed. The effect of gas circulation rate on the oxygen transfer was considered. The model can be used for the determination of mass transfer performance of the airlift reactor in which the whole liquid phase cannot be assumed to be fully backmixed. The simulation results showed a good agreement with experimental results.