In this paper, we carried out uniaxial compression experiments of some gypsum samples with many pre-existing flaws. Some simplified tools of the image processing are utilized, in order to make an observation about the deformation of specimens during experiments. The results enable us to discuss the fracture mechanisms, such as localization deformation failure, in the samples with pre-existing flaws. This result shows that the fracture of them are controlled by both wing cracks and secondary cracks.