Abstract: The video images of specimen surface can be used as an information carrier for the deformation and failure of a rock. In this paper, a quantitative analysis method for video images photographed from loaded rock specimen is proposed to evaluate its damage evolution. The extent of deformation and crack propagation can be expressed by using relative entropy. The rapid increase in the relative entropy is then used as the starting instant of the damage evolution. In the current study, using the video images photographed during the laboratory uniaxial compression test of the granite specimen, the distributions of compositions are determined by the gray threshold segmentation. The relative entropy is used to represent the damage evolution features of the compositions at various locations. The damage evolution process of the whole specimen is thereafter investigated. The relationship between the damage evolution and the types and locations of the compositions are furthermore explored. The results show that the damage of the specimen starts from the middle region and extends successively to the lower-left and upper-right regions, and the damage sequence is feldspar, quartz and biotite with the degree orders of quartz, feldspar and biotite. The biotite damages from the upper-right region and extends to the middle region, quartz damages from the middle region and extends successively to the lower-left and upper-right regions, while the feldspar damages from the upper region and extends successively to the middle and lower regions.