A total of 20 representative sludge samples (6 original and 14 composites) were collected from 6 wastewater treatment plants, for investigating the feasibility of sludge thermal treatment. After a thorough identification of the sludge samples, characteristics of sewage sludge combustion were studied using a bench-scale fluidized bed reactor. To obtain a suitable operating condition, precombustion analysis was made based on both calculation and cold/hot bed testing, to determine the gas flow rate and minimum/maximum fluidized velocities for maintaining a steady fluidization. Critical operational parameters of the fluidized bed reactor in the study were evaluated, including temperature, flow rate of bottom gas supply and total flue gas, resistance time, fuel/air ratio, and sludge-feeding manner and rate. The optimum operation condition was selected to evaluate the combustion efficiencies of 20 sludge samples from analyzing the collected flue gas and ash compositions. Meanwhile, the emission of SOx, NOx, and CO in flue gas was determined, and mass balance analyses of carbon, nitrogen, and sulfur were carried out. Finally, sludge combustion characteristics were summarized, in terms of fuel quality, combustion quality, gas emission, fluidization quality, and material balance.