Optical Microscope and SEM(Scanning Electron Microscope) are used to observe the crack evolution of rock under compression. Two kinds of rock (marble and granite) are investigated. The evolution of crack pattern shows that localization is a key factor inducing brittle failure. Crack of granite readily tends to localize and show more brittle. Contrarily, randomly distributed cracks induced by compression load results into the low brittle of marble. The mechanism underline this phenomenon is that marble has higher heterogeneous than granite. So, the key point to describe the brittle failure mechanism of rock is to determine the micro-mechanical and the damage evolution properties.