Abstract: X-ray CT is an effective nondestructive means to detect initiation, extension and breakthrough of the internal micro cracks in concrete. Using a self-made CT test system, CT images have been obtained for showing the whole process of cracking, crack extension and breakthrough in specimens, via real-time CT scan observation on the microscopic failure process of concrete specimens under static and dynamic loads. Analysis of these images revealed the details of static and dynamic failures, and destructive strength of the specimen is obtained by using the load-displacement curve. CT images and CT numbers show that under static load, cracks develop and extend following the structural weak surface and finally breakthrough in specimen failure. Under dynamic load, the micro cracks generate and develop faster and they follow the path of fastest energy release, causing more cracking of aggregate and hence leading to a full play of the material strength and a higher dynamic strength. This study provides microscopic features of static and dynamic damage of concrete and lays a basis for further study on strength theory of concrete material.