In the damage research before on the CT image of quasi-brittle materials such as concrete, the damage evolution is often described by the change of average CT values or image threshold segmentation method. However, the evolution of average CT values is insensitive to the damage, and the threshold value is always not only, which grimly depends on the researchers’ judgment. Because CT image is a gray image and different grayscale expresses different density distribution, it is tried in the paper to express the variety of material density with variety of grayscale of CT image. A description index of Rd from fractal theory is introduced in the paper to describe the variety of grayscale. Thereby, a damage variable is defined by Rd index, which can be used to study the developing process and evolving rule of concrete damage.