Abstract: Plastic bonded explosive(PBX)can be regarded as a heterogeneous composite material consisting of crystal and additives.In order to investigate the crack propagation in the inner structure of PBX under thermal shock,a series of CT images about the fracture evolution process of PBX under thermal shock condition was obtained by applying the X-ray micro CT technique.This paper presents a method of determining cracks according to linear or circular shadows whether or not appearing in the minus image between CT images.The characteristics of distributional region of cracks are also analyzed by means of changing threshold value.Then,the crack region can be determined based on the CT physics theory.A few issues about crack determination criterion and crack locations are discussed.Finally,whether the crack appearing is determined,and its specific region is quantified in CT image.In theory the influence of additives is eliminated by this method.