A fluidized bed furnace with a 0.7 m square sand bed was operated with air mass velocity of 0.19 to 0.6 kg/m(2) . s, compared to ordinary rates of 0.20 to 0.25 kg/m(2) . s. The air ratio was adjusted over a wide range of 0.62 to 1.50, compared to ordinary rates of 1.3 to 1.5. From the results of these experiments, the combustion efficiency at sand bed decreased as cake load, fuel load and air mass velocity increased. The combustion efficiency is proportional to the air ratio, therefore the combustion efficiency decreased as the air ratio decreased. A definite correlation of combustion efficiency and sand bed temperature was not observed in the range of 873-1,171 K under the self combustion condition. At a high air mass velocity, the operation of the high cake load can be accomplished with a high air ratio. And at high cake load the freeboard combustion was superior to sand bed combustion.