A novel fiber optic probe system has been developed for studying the bubble behavior in gas-liquid-solid three-phase circulating fluidized beds (TPCFBs). Mathematical method to analyze data by probe technique has also been discussed in this paper. The bubble size and its distribution, bubble Sauter diameter, gas-liquid interfacial area and bubble rise velocity, have been experimentally studied using the fiber optic probe. The experimental results show that the bubble size distribution in TPCFBs follows lognormal function. The bubble Sauter diameter has the radial profile with smaller value in the central region than in the near-wall region, different from the conventional three-phase fluidized beds (CTPFBs) without outer particle circulation. The distribution of bubble rise velocity, gas-liquid interfacial area and effect of the operation conditions have been experimentally studied.