The effect of liquid viscosity on hydrodynamics and bubble behaviour of an external-loop airlift reactor within air-water system were studied by an electrical conductivity probe. Carboxyl methyl cellulose (CMC) was used to change liquid viscosity (1.00-51.25cP). Gas holdup increased with superficial gas velocity increasing, as well as bubble Sauter diameter and bubble rise velocity. With liquid viscosity increasing, average gas holdup increased first then decreased, while bubble Sauter diameter was opposite, with critical viscosity mu(l)approximate to 3.7cP. The cross-sectional average bubble Sauter diameter increased obviously after mu(l)>3.7cP, and its size distribution was wider within the higher viscosity. Bubble rise velocity increased significantly and nearly unchanged after mu(l)>10.3cP. Gas holdup first increased then remained unchanged with the axial height increasing. The radial profile of local gas holdup presented a central peak distribution, and the value at inlet was smaller than that at the higher position. Average gas holdup was correlated greatly with superficial gas velocity and liquid viscosity.