The green bulk amorphous silicon nitride was prepared by cold isostatic pressing without the presence of sintering additives, using an amorphous silicon nitride as raw powder. The crystallization behavior of amorphous silicon nitride was investigated under N2, Ar atmosphere and vacuum using X-ray diffractometry (XRD). The effect of atmosphere on the process and the end products of crystallization of amorphous silicon nitride was studied. The single phase beta-Si3N4 and Si2N2O could be prepared in the different sintering atmosphere as the end product. The possible reaction process was discussed also.