Abstract： Objective To study the dynamic change of anatomy and dosimetry distribution and its influence during intensity modulated radiation therapy(IMRT) for nasopharyngeal carcinoma(NPC). Methods From June 2006 to August 2007,12 patients with stage Ⅲ-Ⅳ NPC receiving initial IMRT concurrently combined with chemotherapy were included in the study. The target volumes and,involving organs were contoured on the first set of spiral CT images. When IMRT planning was finished,a second CT scans was acquired to rectify the treatment center. Weekly spiral CT images acquired during the treatment period according to the same treatment center were fused with the first CT images. In order to determine the best time of replanning, modified plan target volume( PTV1 ) and normal organs( parotids and outline) were contoured on the fusion interface by a single physician. Changes of each structure throughout treatment period were measured by a system software. Then the dose distributions were computed and evaluated for replanning CT using the same beams arrangement in the original plan. Cumulative dose was estimated compared with the original plan. Results The volume of outlines and parotids decreased gradually, and the change came to peak in the fifth week. So the fifth and first week CT scans were selected as research objectives. No significant changes in maximal and mean dose was observed in the brainstem, spinal cord or mandible. Despite volume changes,D99 and D95 of the PTV1 did not change siguificantly(P＞0.05). D95 of the bilateral parotids increased significantly ( PL = 0.03,PR = 0.01 ). Conclusions During IMRT for NPC, the volume of PTV1, the outlines and parotids decrease,and the change come to peak in the fifth week. Comparing to the first treatment plan, the dose parameters of the parotids increase significantly in the second plan based on the fifth week CT,but those of the brainstem,cord,mandible and PTV1 change slightly.