The bubble size and its distribution has been studied in three-phase circulating fluidized beds by using the newly developed fiber optic probe system systematically. The experimental results show that the distribution of the bubble size can be described by the lognormal distribution function. The mean bubble size in the center region of the fluidized bed is smaller than that in the region near the reactor wall. The mean bubble size decreases with the superficial gas velocity and increases with the particle-circulating rate. The superficial liquid velocity has almost no effect on the mean bubble size.