Abstract: Numerical compression specimens with different brittleness are constructed based on the UDEC-Voronoi model. The number of cracks and the crack density are monitored by the FISH programs. It is shown that the crack initiation stress increases with the increase of the brittleness index. However, the crack density at the crack initiation stress decreases. That is to say, the crack initiation of high brittle rocks is at a higher stress and with a fewer cracks as compared with the low brittle rocks. Furthermore, the peak stress increases as the brittleness index increases. A fitting formula is established to estimate the crack density at the peak stress against the brittleness index, and the brittleness index 0.6 can be used as an empirical value for judging the shale brittleness.