In other to obtain information on the behavior of the external-loop airlift reactor operation with air-water mixtures, the gas holdup, overall volumetric oxygen mass transfer coefficient, vertical bubble length and bubble velocity were studied. The effects of gas velocity(0.02-0.18m/s)and the length of horizontal connection pipe(0.1-0.5m)with respect to the above mentioned parameters have been investigated. The results showed that the length of the horizontal connection pipe was a dey design parameter for the external-loop airlift reactor. The bubble rising velocity in the riser and bubble downcoming velocity in the downcomer increased due to increase of the circulation liquid velocity as the horizontal connection pipe length was increased. The downcomer gas holdup and vertical bubble length in the downcomer decreased due to increase in gas-liquid separating abilities of upper connection section as the horizontal connection pipe length was increased. The overall volumetric oxygen transfer coefficient in the external-loop airlift reactor with a short connection length was higher than that with a long one due to higher gas holdup.