Cracking of a cristobalite-containing mullite body at the middle stage of crystallization during cooling was investigated by acoustic emission (AE) and the crack formation was discussed. The specimen was composed of an outside layer of crystalline cristobalite and the inner part remaining as a glass phase. There are two kinds of cracks, of which the one is almost parallel to the surface of specimen at the boundary between the outside layer and the inner part and the other is perpendicular to the surface. AE signals indicated that the cracks occurred in two stages, the first stage at the temperature range 750-650°C and the second stage below 250°C. Cracks parallel to the surface were formed at the first stage due to a mismatch in thermal expansion coefficient between the outside layer and the inner part. Those perpendicular to the surface were generated at the second stage under a tensile stress developed in the outside layer, which was caused by the β-α phase transition of cristobalite, in the direction parallel to the surface