Characteristics of pressure fluctuations and bubble size were investigated in the riser of a three-phase circulation fluidized bed bioreactor with viscous liquid medium, whose diameter is 0.102 m (ID) and 3.5 m in height. Effects of gas (0.01-0.07 m/s) and liquid (0.17-0.23 m/s) velocities and liquid viscosity (0.96-38 mPa.s) on the bubble size in the riser were examined. The bubbling phenomena in the bioreactor with viscous liquid medium were interpreted effectively by measuring and analyzing the pressure fluctuations by adopting chaos theory. The bubble size increased with increasing gas velocity or liquid viscosity, but decreased with increasing liquid velocity. The bubbling phenomena became more complicated and bubble size distribution tended to broad, with increasing gas velocity or liquid viscosity. The bubble size was well correlated in terms of correlation dimension of pressure fluctuations as well as dimensionless groups within these experimental conditions.