The Alumina powder mixed the each organic binder(A, B, C) were molded rod like test pieces by a screw injection-molding machine. The binder(A) consisted of polystylene (PS), polybutylmethacrylate(PBMA), stearic acid(St) and paraffin wax(WAX); (B) consisted of PS, PBMA, St, WAX and atactic polypropylene(APP); (C) consisted of PS, PBMA, St, APP. The test pieces which were made of the binder(B), (C) cracked easily during the debinding process, but those which was made of the binder(A) didn't crack. From the above fact, one of the reasons was shown that owing to the radical thermal decomposition of the APP at the about 200°C, the rate of debinding of the binder(B), (C) were rapider than that of binder(A). So that it was clear that the causing cracks within the test pieces resulted from the rising rate of debinding at about 200°C.