Abstract：Rock density damage increment with CT can be obtained by subtracting CT number of the undisturbed rock samples from that of the loaded rock samples. However，the special phenomenon may come into being when density damage increment data are translated into grey images. In such CT images，the circular contour line of rock samples is divided into two half circles. The half circle is white，and the other one is black. Such contour line of rock samples is called half-white black circle in this paper. In order to discover the information mechanism of the phenomenon，a group of CT test data，which are from the same scanning section of a sandstone sample at the four different stress steps，are used to analyze the CT number distribution. The analytical results show that there is the small offset between CT images，which is the reason that the half-white black circle phenomenon can be found in density damage increment images. After the small offset is corrected by registration processing，this phenomenon disappears.
