Abstract: The development of radiotherapy is closely linked with modern medical imaging technique. 4-dimensional CT increasingly becomes an effective measure due to the improvement of temporal and space resolution of CT imaging. In the simulation of radiotherapy, 3-dimension images in different phases can be collected with respiration-gating technique. Therefore, a precise mathematical model is set up after the rule of tumor motion along with respiration is found out, which is significant for the improvement of treatment planning. Moreover, two orthogonal X-ray imaging systems mounted on the gantry of a medical linear accel-erator for tracking the real position of tumor is also an interesting aspect in 4-dimensional radiotherapy study.