Abstract: CT scanning tests of cement paste with water/cement ratios of 0.3 and 0.5 from 1-40 days' age were carried out. The 3D microscopic model of cement paste was successfully reconstructed, and the morphological changes of unhydrated cement particles, hydration products and pores during this period were observed. Based on the results of CT scanning, the volume fraction of unhydrated clinkers and the hydration degree were calculated. The calculated results were compared with the commonly used TGA method, and agree well with the TGA results. CT scanning could be used as a reliable method for quantifying the hydration degree of cement.