Abstract：Utilizing the evolution images of crack or damage in rock mass and soil offered by computerized tomography(CT) technology，the changing value of CT number at each pixel is defined to be the maximum of absolute value between the margin of the pixel CT number and that of the proximate pixels in the same row and column respectively. According to the distribution of CT number on scanning section，the changing criteria of CT number at pixels where x equals 20 are defined；and then the doubtful crack is obtained. Simultaneously，the geometrical and CT information in the region of crack is computed. Then the length and width of crack are defined and calculated. The work is important to advance the quantitative analysis of CT image in geotechnical engineering forward.
