Abstract: 4D CT imaging technology,which could track the movement of the tumor in the entire respiratory cycle and reduce motion artifacts effectively,has been widely used in radiation therapy for tumors.The current 4D CT methods require external surrogates of respiratory motion obtained from extra instrument.In the paper we propose a novel automatic 4D CT sorting algorithm that completes 4D imaging without these external surrogates.The first step of the algorithm is to collect the image data within a cine scan protocol.Beginning with the first couch position,images from the adjacent couch position are selected out according to spatial continuity.The process is continued until images from all couch positions are selected for the entire 3D volume is produced.The algorithm is verified by respiratory phantom data and clinical image data are used to verify the algorithm.The primary test results show that the 4D CT images created by our algorithms have eliminated the motion artifacts effectively and clearly demonstrated the movement of tumor and organ in the breath period.