Cracking is one of the major distress types of asphalt pavements. Expected life of pavements, pavement condition and maintenance cost are directly related to pavement cracking. It is necessary to promote the understanding of cracking mechanism of hot mix asphalt (HMA) based on fracture mechanics. Simulation, surveying, observation, and measurement of cracking in pavement structures have been reported in literature in last three decades. However, cracking process in asphalt mixtures in a three dimensional scale is still a great challenge to road engineers. This paper reports a research effort of dynamic observation of the micro-fracture process of asphalt mixture under compressed condition using CT (computerized tomography) method. Through CT scanning, the clear CT images, which include the microcrack compressed, growth, bifurcation, development, the crack fracture, and the asphalt concrete sample failure were obtained. The CT numbers, CT images and the other data were analyzed. Analysis of the relationship between of CT number and volumetric strain suggests that damage value at zero volumetric strain can be used as threshold point in damage analysis. Corresponding stress value can be used as maximum allowable stress in design practice.