Although there has been much research of cracks of the cement-based materials using optical and electron microscopy two-dimensional (2D) imaging methods, the real three-dimensional (3D) crack shapes have not previously been revealed. Thanks to the focused ion beam (FIB) tomography and the follow-up image processing, two 3D subsurface cracks and a cluster of inner cracks were picked out and discussed in this research. It was found that the subsurface crack (its length is about 15 part, width about 1-5 prn, and opening about 1 ~tm) was much larger than the inner crack (its length and width are about 1-5 pro, opening is from 200 nm to 1 pan), which arose from the sample preparation process. Besides, it was revealed that most of the inner cracks were in the form of clusters.
