Objective: To evaluate intrafraction mobility and its effect on the target volume and the normal tissues by the cone beam CT(CBCT).Methods: A total of 23 patients with thoracic esophageal cancer were enrolled in this study.CBCT scans were performed and registered automatically to planning CT scans before and at the end of irradiation.The setup error was corrected before the treatment, and mobility data during the irradiation were measured as the intrafraction mobility.Plan 1 was the original treatment plan.Plan 2 was generated as follows: if the intrafraction mobility in any direction was 3 mm or more, the fraction would be simulated, and the rest of the prescription dose would be still performed according to Plan 1.Plan 3 was defined as follows: if the intrafraction mobility in any direction was 3mm or more, the simulated fractions would be distributed evenly to the total fractions and prescription dose.The dosimetry distribution of the intrafraction mobility against target volume and organs at risk were analyzed.