The material balance in CFB furnace has been proved important for the performance of the boiler. The bed inventory of CFB boiler is composed of fuel ash and sorbent additives. It is necessary for the engineer to know the ash size formation character of fuel before make a proper design of the boiler and the auxiliary system. Traditionally, the only way to know the ash formation information is by the fuel combustion test in pilot CFB furnace. Because of the high cost of the test and the insufficient information from the test, such test was rarely done. The present investigation is proposing a simple laboratory test procedure instead of the pilot CFB combustion. The validation of the method is introduced in present paper. Two kinds of coals include the coal from Gardanne 250MW CFB were tested by present method and the result is compared with the data from Gardanne boiler.